For HRE:
If the Germans are split into Bavaria, Saxony, and Brandenburg then the HRE mechanic could work this way: (besides the other previous suggestions made)
As soon as the HRE is formed in 962, all 10th century provinces historically a part of the HRE become HRE province (all cities within those provinces show that flag from RFCE++). Such areas include: Germany (excluding Holstein), Austria, Carinthia, Northern Italy (excluding Verona), Netherlands, Lorraine?, Silesia, Bohemia, Corsica, Sardinia, and Provence. All civs with cities within such provinces are automatically in at the start. An election is either held for the first emperor or one is chosen at random... Before 962 all civs are essentially standard.
The mechanics I thought of along with RFCE++'s
-Collecting taxes, raising an army, adding/excluding provinces from the HRE: I would adjust the last one so that only the elected Emperor can unilaterally add a province or take one away. And using these primary options excessively should make the AI more likely to vote in an Emperor from a different civ.
-voting is based on population within HRE provinces. Outside ones do not count.
-Common war: emperor holds a vote to declare war on a non HRE civ. less likely to be approved if at war, or war weariness high.
-Members can secede from the HRE at any time (through what I'm imagining as an HRE button somewhere, or a diplomatic option with the emperor) at the cost of being at war with everyone in the HRE. For example: Italian civs secede and become at war with all the other HRE civs.
-Automatic defense pacts: If a non HRE civ declares war on a HRE civ then all HRE civs declare war on the aggressor with an exception below.
-If you control any cities in a non HRE province then the automatic defense pact doesn't apply to you in any way. Example: Burgundy's capital and Lyon are not part of the Emperor's sovereignty (unless an addition was made prior), therefore if Bavaria is declared war on by France Burgundy doesn't automatically declare war on France. However, if France declares war on Burgundy then Burgundy is on its own! *This prevents a giant foreign civ from having a toehold within the empire and gaining all of its benefits.
-If you are a part of the HRE you can declare war on any other civ without any penalty unless it's with the HRE.
-So this system is somewhat constraining for an expansionistic player, so maybe the automatic defense pact doesn't apply to infighting members?
-This helps smaller states prevent from being overrun so easily like Bavaria, Brandenburg, Saxony, and Lorraine.
Downsides: I don't know how this can affect vassals (maybe all the automatic defense pacts dissuade HRE civs from trying)? And it's complicated.
Although splitting Germany doesn't need a huge protection system. Saxony is very large, and Bavaria can have three strong cities in the south. Brandenburg would be susceptible to vassalage though.
***So an HRE system may not need to be implemented!
This is more speculation, but if the map is extended there could be a possible addition of the Khazars (along with the Armenians and Georgians). SOI has plenty of info for the last two, but not the first. So no harm in suggesting it.
The Khazar Khaganate
Spawn: 618 at Atil or Balanjar with three settlers, three archers, one spearman, five or six UUs
Leader: Tong Yabghu, Bulan, or Benjamin
UU: Ursiyya - replaces lancer. +1 strength, +25% vs mounted units, and +1 movement. Cannot receive flanking promotions. (Use artwork from RFCCW)
Or Tarkhan - replaces lancer. +1 movement, +25% vs mounted units, and costs a little less.
UB: Tudun - replaces manor house. -20% maintenance cost of city.
Or Yarmaq Mint (from RFCCW) replaces market. +10% extra wealth
UP: Pax Khazarica - Your positive stability provides extra income during the early Middle Ages.
Or Sacred Diarchy - Half combat experience is required for great generals to appear.
Or Sacral Diarchy - Two capital cities may be built? Something like that...
UHVs:
1. Convert to Judaism by 900 (Or Judaism could have a high rate of spreading to the area)
2. Control the Caucuses, Khazaria, Crimea, Kuban, and the provinces that make up the Ukraine (East of Moldavia) by 850.
3. Control thirteen provinces by 800
4. Conquer Jerusalem by 1080
5. Acquire the following resources through trade or conquest: marble, silk, incense, and silver by 840
6. Ensure that there are no European cities in the steppes (Khazaria, Kuban, Crimea, and southern Ukraine) in 1040.
7. Have at least one city with Judaism
8. Have 4,000 gold by 970.
9. Have Islam, Christianity, and Judaism spread in Atil by 920
10. Have open borders agreements with five civs in 1000
Potential Dawn of Man texts
Florence "Margravine Matilda has passed away, and with her the feudal Tuscan domains that ruled over us. The Florentines support a republic ruled by Florentines! With the empire and the pope to our north and south respectively, our fledgling republic may have a difficult road ahead at retaining independence. We will prosper not through conquering our enemies, but through banking and finance. Our city will become the financial capital of Europe!"
Navarre "Your lands have resisted the tide of moorish invaders from the south for over a century, and after the battle of Tours their power has gradually been receding.
The combined strength of the northern Christian states can drive the heathenish Moors back over the straits of Gibraltar and reclaim Iberia for the cross! With our small towns in the hills of Basque Country it will be wise to divide the spoils and be weary of our much larger neighbors."
Sicily/Naples "Our Norman army hears of great news. Our fellow countrymen have been serving as warlords and mercenaries to various Lombard and Byzantine units in the rich southern Italian countryside for years. Without a central authority, the divided land lies fragmented and weak for conquest under one king, Robert Guiscard. With even more good fortune the recently divided, but rich, island of Sicily under its weak emirates will fall into our possessions. Not even the Byzantine Emperors will be able to stop us!"
Anglo-Saxons "Britannia has succumbed to wave after wave of Jute, Angle, and Saxon invasions after the Romans abandoned the island. Slowly these tribes have organized into small states capable of self governance. On this island your state will be undisturbed for some time, but not isolated. Beware of invaders from the North Sea and Northern France."
Khazars "Nomads have roamed the steppes for centuries, but only the strongest hordes can forge lasting empires. The Gokturks from the Asian steppes have lost their hold on European lands as their empire fractures into feuding tribal factions. This leaves our forming empire as the strongest in the region with control of the northern Silk Road route. With cavalry that never retreats and an expansive trade network, we will forge an empire to last for centuries!"
Respawns(If they are turn based)
Florence 1197, 1378, 1512? (Too much too late?)
Milan should have plenty from RFCE++
Navarre 1004
Sicily/Naples none 1200
Anglo-Saxons none
Bavaria one during the 15th and 16th centuries?
Saxony one during the 15th and 16th centuries?
Brandenburg 1356
Leon 1296 only if Castile is collapsing, or core is controlled by indie cities.
Khazars 750
Castile same as Spain
Dynamic names extra:
If Khazars are a vassal to Byzantine: East Tourkia.
If Sicily/Naples is a vassal to (capital in Southern Italy) Byzantine: Catapanate of Italia.
If Sicily/Naples is a vassal to (capital in Sicily) Byzantine: Theme of Sicily.
Duchy of Bavaria should be Bavaria's main name, assuming Germany is split.
If Florence is a vassal to Germany: Margraviate of Tuscany.
If Florence is a vassal of Byzantines: Exarchate of Florentia/Tuscia Theme
If Florence is a vassal of Ottomans: Eyelet of Floransa
If Kiev is a vassal of Khazars, dynamic name becomes Tributary Rus' States
Add plenty of horse archer barbarians for Khazars
Edit:
Ok, I know having a more involved papacy would be great but a playable pope seems... ridiculous. However, I'll create a case for it because there's no harm having it on the table in a more developed light for something someone may do in the future.
Papal States:
Leader: The Pope (or Urban II, and Pius II)
Spawn: Rome (everything as is)
UU: Swiss Guard - same as mercenary
Or Apostle (replaces Catholic Missionary) An invisible unit that can spread Catholicism, or purge a non religion. The purge option requires open borders with owner.
UB: The Saint's Tomb - replaces Manor House. Costs 300 hammers? Bonus is +4 culture, +3 great saint points, +1 happiness.
UP: Crusades - Can call and direct crusades
Or Relics of the Saints - Relics produce extra income in Rome (say 4 gold per turn?)
Or same name - settled great saints produce double wealth benefit.
UHVs: for an active or passive pope
1. "Roman Splendor" Make Rome the city with the highest culture and wealth in 1200
2. "The Holy See" Ensure that every European city has Catholicism
3. "The Savior" Conduct four successful crusades
4. "The Inquisition" Purge all non Catholic religions from every European city
5. "His Holiness" Become a great saint by year X
6. "One God. One Church" Ensure there are no Orthodox civs alive
7. "Sacred States" Control two cities in Central Italy and one in Provence (Avignon)
Notes: the Papal States can be implemented as playable either as an OCC with religious/diplomatic emphasis, hence the UU option, or more militarily as any standard civ, but with plenty of restrictions. All provinces with the exception of Provence and Latium should be foreign core. War map and settler map should be small obviously. It should be very difficult for pope to hold extra cities outside of Latium.
The crusades UP is tricky to say the least. A screen comes up with three predetermined cities to send an army to during specific turns(Jerusalem, Antioch, Constantinople, Kaunus, etc). As pope, you choose city and who leads the crusade. If you don't pick the one who obtained the most votes, then maybe one diplomatic penalty that lasts until next crusade. A successful crusade gives pope extra faith points?
*There are plenty of pitfalls: What if Rome builds a navy/army? What if Rome collapses? What if their culture radius grows too big? What if Rome becomes someone's vassal? What if youtien plays as the Papal States?
1. I don't think larger nations should have to worry abut such a thing happening.
2. Immediate respawn? For the player, lose half wealth and culture.
3. Keep radius of all papal cities confined to basic cross
4. Then they influence pope?
As for the last one, we can only pray
*I do like the Sengoku mod by embryodead where expansion counts as a diplomatic penalty against other expansionistic clans. Ex: "Your expansionistic practices trouble us!" To adjust this feature for RFCE make historically large civs a little sensitive to the player's expansion. Expansion civs: Russia, Austria, Ottomans, etc. Smaller/stagnant civs: Genoa, Byzantium, Bohemia, etc. This would keep large nations from being overly friendly with one another and create competitive wars. Hopefully smaller civs can use diplomacy to evade such costly wars... However, this may unbalance diplomacy!
Edit II
If the map is extended eastward, maybe cut the western parts of the map that are mostly ocean tiles and islands (18 tiles). Readjusting Portugal and Norse UHVs would be necessary though.
Reiterating on companies: I think having companies modeled like SOI's will work historically speaking, and for gameplay. I don't know how viable this switch would be because I can't mod. Every company will spread to a city primarily based on history within the province, tax rate, and resources needed; secondly (which will vary) peacetime, war, trade routes, culture, civics and so on. For example, the higher your tax rate then the spread of any company into your lands will decrease, and if too high then they leave. If your rate is low, then the spread rate is normal or much higher depending on how it is coded.
The guilds civic negates all corporation effects. The apprentice civic could either increase the spread rate of companies or improve the yield. Divine Monarchy, Despotism, Theocracy decrease spread. Merchant Capitalism, Mercantilism, and Trade Economy increase spread.
Maybe rival companies can cause a rare one turn revolt if they occupy one city (low probability per turn).