Not sure if my list below is for a patch or for a future expansion!!...
(sorry, sorry, sorry if what I suggest has already been proposed many times! Reading part of the forum, I saw nothing articulated in the same way, but this forum is sooooo big!...)
I-Vassals on their own will
a- A vassal will not start a war and push the master into war against will. Can not declare war without first denunciating vassal treaty with master. Master has the choice between accepting or refusing the end of vassalage (hence supporting all consequences).
b- War declared on a vassal by another civilization: the master has the choice, under penalty of gold, units or research to the attacked vassal, to stop the vassalage agreement and not join the war. Penalty of units means that the master must provide the vassal with a certain amount of military units (gift) for
c- under penalty of gold (proportional to the number of “research” points needed for this discovery), a master can directly change or decide the research of a vassal. In this case, as the “fine” has been paid, the master inherits also of the research as soon as it is found by the vassal.
d- if they propose to be vassal, all bad red diplomatic points are reset at 0, against a fee proposed to the master (research, gold, goods).
II- Vassals, after capitulation
a- under penalty of gold, a master can directly change the research of a vassal. In this case, as the “fine” has been paid, the master inherits also of the research as soon as it is found by the vassal
III- All Vassals
a- may not trade with civilizations that are not already trading with master, i.e. all vassals
must and can only trade with the same civilizations already trading with master. A new vassal denunciating a trade agreement with a civ that is not “registered” with the master, does not have any diplomatic penalty with this civilization. This includes goods and all treaty (including open borders).
b- joint work force (perhaps already existing!!): automated workers from vassals and masters improve all tiles, following options set by the master (if the player), i.e. forest, old improvements, and do as if master and vassals are the same civ.
c- Vassals can be specialized in military, growth/economy, research, culture: the master can order a vassal to choose a development path that is consistent with her/his strategy. Some vassals will be more aggressive, others more mercantilist, etc…
IV- Neighbors
a- new treaty: exchange of work forces. Idle workers from both contiguous countries can work tiles of the other country, automatically and according to working rules adopted by the country where the work takes place (chop or not forest, etc.). A worker from a foreign country beneficiates of the technology of the host country, the time the worker works into this country. This treaty can have 2 levels: exchange of idle workers, or rent of workers (idle or active): workers are then treated like goods. At least Slavery must be known.
b- Any attack done by you or another civ
from inside another foreign border of a third civ triggers diplomatic and/or military retaliation against the invader/attacker or the victim (depending on diplomatic relations). Example: you have a city just at the border with Bismarck. Bismarck has neutral agreement with you but open border with the attacker Shaka with who you are at war: Shaka attacks your city from within Bismark’s border but you can not reply at it would mean declaring also war on Bismarck!!
All this depend on agreements with the third party: you are at war against B, you are attacked from territory C
case 1: C has open border with B but not with you. C must choose:
…allowing the attack, in this case C must declare war on you.
…cancelling the attack from B: C cancels then the open border with B (at the risk of deteriorating badly relations with B!...)
case 2: C has open border with B and also with you. C can not choose and all attacks from within C’s border are totally forbidden
case 3: C has open border with attacker B but C is a vassal of another civilization. C can not choose and case 1 or 2 are applied according to the status of the master with you (master has open border with you or not)
exception: rule does not apply to submarines, BUT applies to bombers/fighters when the air path must cross a neutral area for attacking in direct line: range is then decreased if aircrafts must go around a neutral territory.
New improvements
a- border posts: can be built on any border tile (except mountain). Act as a fort and suppress other improvements; increase trade outcome of a tile (+3?); add +1 visibility to units in this post; bonus against barbarian; can not be used or crossed by spies. Needs: Currency & Code of Laws
b- custom offices: in all cities with airport or with harbor. Add 50% of trade route incomes; add +1 population but also + 1 unhappiness and +2 unhealthy (afflux of immigrants)
c- new World Wonder: NGOs (Non Governmental Organizations, for helping defavorised countries with low-paid highly-skilled voluntaries): It gives + 6 NGOs (like “Hits”

; benefit: all “bad log” with a civilization receiving a NGO is reset (ex.: the “you have refused to trade with us” that you keep against you for ages!!!), or at least is reduced by -6 for each NGO. We do not add more green points, but decrease the red bad points. Technology needed: Medicine
New trades
a- Bad red diplomatic points can be reduced by paying damages/interest to the civ, in gold, research, goods, or even cities or units. Each red line can be negociated (it is not a bulk negociation for all red points). Needs: Currency, Code of laws and Guilds.
b- Workers: possible to buy (not rent or exchange like for neighbors treaty) and worker is 100% yours. Needs: discovery of Slavery, Guilds
b- new resource: Salt, available after discovering equitation. Obsolete with discovery of Industrialization. This resource gives +1 gold and +1 happiness (or +1 hammer).
New behavior for units, new units
a- spy, or : can also add poison or terrorist acts (reduce health, happiness and population). Benefits: add +10 green diplomacy points with all civs that are annoyed or furious with the attacked civ. But add -10 to all civs friends or pleased with the attacked civ, and -4 to all neutral civs (cautious), and 0 to all civs unknown yet. This can only be done if civic is with a State Religion or Police state.
b- Explorer: can not be killed by barbarians. Explorers are almost pointless units. In huge maps, where barbarians are making cities on undiscovered continents, explorers can do nothing as they are immediately destroyed. This would give an advantage over scouts, and offer them a better role in a game.
b- Diplomat: produced like missionaries (max 3). When stationed in a capital, help reducing the bad red diplomatic points turn after turn (ex: 1% less red point each turn a diplomat is present in a capital). Cannot be killed except by Barbarians. Behave like a spy in city at war but can not do spy mission! Can not be moved during war time in countries at war with you: must stay in position during the time of the war, except if you require, by diplomatic screen, the diplomat to be repatriated: in this case, you have an instant -5 red bad diplomatic point added to your enemy log against you.
New leaders
a- Great Barbarian: especially with games with large/huge maps, some continents can be populated with many barbarian cities.
…If more than, say, 4 barbarian cities are created in a continent, and your civ is in contact with one of those cities, a diplomacy screen exists with a Great Barbarian
…no civic proposal, no goods proposal, no treaty except cease fire (no Peace Treaties)
…can only be bribed with gold or a military technology (no growth, no economic or scientific) or military units or cities
…can propose slaves (workers), maps and cities for sale and at high price (for just giving the feeling that it is better to destroy them than deal with them!).
… they can also propose skilled explorers for hire. (see modig for Explorers)
…The Great Barbarian can NOT be contacted at will via diplomacy screen, he/she (walkiry) contacts you when he/she feels the needs, and only when you have a unit against a Barbarian cities (barbarian units in the wild are just illiterates…

. Especially useful on large/huge maps and marathon play!!
b- FREE leader: a generic leader, available in Custom Games only, with Sid or Soren’s face or a female pretty-but-tough face (!), and you choose the associations (philo, aggress, fin, cult, etc…

AND the starting 2 tecs.
…Other specifications are taken randomly from another civ (special building/unit, color) at game start, or can be also choosen by player (in this case, take the whole civ spec: special unit+color+civ name+city names).
…For choosing this customized leader, game difficulty can NOT be less than Monarch (or perhaps Emperor)! This allows what-if games, and test the highest challenges for those who can not do it with a normal game (99% of us??...)