Well I think that's why we are planning stuff out now. So we can put lots of stuff in that all "play nicely" with each other. For example the Prehistoric Era stuff would ideally not effect the game play if you had started in the Ancient Era. However I think there should be some incentive to to play the Prehistoric Era. Perhaps it can give you some special tech or prerequisites for later things you could not otherwise obtain if you skipped that Era.
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Anywho I had another idea. I always loved the Assimilation feature in Civ4 AND where you get the special buildings and units that the city you took over had. However now we have the whole city state thing. Back when I was modding for Civ4 AND I was trying to implement a system where if you made a wonder you would get a naive tribe. Much like the Native Embassy system set up in "Age of Empire 3". However this city state system is even better!! If you take over a city state then depending upon what city state it is you can produce special units, buildings and maybe even gain access to hidden techs!
Here are some examples from "Age of Empire 3". Note that icons would be already created from these and possibly even stats! I also tried to get it as close to the original game as possible but some bonuses had to be changed for Civ. Feel free to change them or think of new ones.
City States as Native Tribes
Apache
Special Unit: Apache Cavalry (Replaces Calvary)
Techs:
- Apache Cactus (+1 Food from Desert tiles)
- Apache Raiders (200% More Gold from Pillaging)
- Apache Endurance (Apache Cavalry get +1 Movement)
Carib
Special Unit: Carib Blowgunner (Replaces Archer)
Techs:
- Carib Kasiri Beer (+10% gold from Brewery)
- Carib Garifuna Drums (Archers do 100% more damage vs workers)
- Carib Ambush Party (Explorers gain +50% attack)
Cherokee
Special Unit: Cherokee Rifleman (Replaces Rifleman)
Techs:
- Cherokee War Dance (20% easier to gain relations with city states)
- Cherokee Basket Weaving (-50% cost of Markets)
- Sequoyah's Cherokee Syllabary (Get a free Settler)
Cheyenne
Special Unit: Cheyenne Rider (Replaces Horseman)
Techs:
- Cheyenne Hunting Grounds (+10% more Food from Deer Resource)
- Cheyenne Horse Trading (Calvary build 40% faster)
- Cheyenne Fury (Cheyenne Rider do +100% more damage vs Calvary)
Comanche
Special Unit: Comanche Horse Archer (Replaces Horse Archer)
Techs:
- Comanche Horse Breeding (Calvary +10% HP)
- Comanche Trade Language (+10% more foreign trade)
- Comanche Mustangs (Comanche Horse Archer get +1 Movement)
Cree
Special Units: Cree Tracker (Replaces Scout), Cree Coureur des Bois (Replaces Settler)
Techs:
- Cree Textile Craftsmanship (10% more Gold from Sheep Resource)
- Cree Kinship (Cree Tracker cost 50% less to build)
- Cree Tanning (Infantry and Calvary get +5% more HP)
Huron
Special Units: Huron Mantlet (Replaces Warrior)
Techs:
- Huron Fish Wedding (10% more Food from Fish Resource)
- Huron Sun Ceremony (Huron Mantlet build 50% faster)
- Huron Trade Dominance (Produces a free Huron Mantlet every 10 turns up to 300 turns)
Incas
Special Units: Incan Bolas Warrior (Replaces Archer), Incan Huaminca (replaces Spearman)
Techs:
- Incan Road Building (Roads cost 50% less to build)
- Incan Metalworking (10% more Gold from Gold and Silver Resources)
- Incan Chasquis Messengers (Scouts get +1 Movement)
Klamath
Special Units: Klamath Rifleman (Replaces Rifleman)
Techs:
- Klamath Strategy (Klamath Rifleman do 25% more damage)
- Klamath Huckleberry Feast (+1 Food to all cities)
- Klamath Work Ethos (Workers work 5% faster)
Mapuche
Special Units: Mapuche Ironwood Clubman (Replaces Warrior)
Techs:
- Mapuche Ad-mapu (All Units cost are 20% cheaper to produce)
- Mapuche Tactics (Melee have +50% vs Siege Weapons)
- Mapuche Treaty of Quillin (+100 gold every 20 turns up to 300 turns)
Maya
Special Units: Maya Holcan Spearman (Replaces Spearman), Plumed Archer (Replaces Archer)
Techs:
- Mayan Cotton Armor (+20% HP for Melee)
- Mayan Calendar (+50% Food from Farms)
- Mayan Astronomy (Astronomy tech research cost 50% less)
Navajo
Special Units: Navajo Rifleman (Replaces Rifleman)
Techs:
- Navajo Weaving (+10% more Gold from Cotton Resource)
- Navajo Shepherds (+10% more Gold from Sheep Resource)
- Navajo Craftsmanship (-10% Maintenance)
Nootka
Special Units: Nootka Clubman (Replaces Warrior)
Techs:
- Nootka Bark Clothing (Workers get +50% HP)
- Nootka Potlatch (Nootka Clubman build 50% faster and 10% cheaper)
- Nootka Woodcarving (Fishing Boats cost 50% less to build)
Seminole
Special Units: Seminole Sharktooth Bowman (Replaces Archer)
Techs:
- Seminole Guerrilla Wars (Seminole Sharktooth Bowman +300% vs City Attack)
- Seminole Bowery (Archer attack +35%)
- Swamp People (Seminole Sharktooth Bowman get no movement penalty for Marshes/Bogs)
Tupi
Special Units: Tupi Blackwood Archer (Replaces Archer)
Techs:
- Tupi Poison Arrow Frogs (All archers +10% Damage)
- Tupi Animal Lore (100% vs Animal Units)
- Tupi Forest Burning (Workers clear/chop forest or jungle 50% Faster)
Zapotec
Special Units: Zapotec Lightning Warrior (Replaces Warrior)
Techs:
- Zapotec Food of the Gods (Farms produce 10% more food)
- Zapotec Cult of the Dead (Zapotec Lightning Warrior do 20% more damage)
- Zapotec Cloud People (Hills give +1 Production)
There are more for the minor Asian and European civilizations but I will get to those later.
EDIT: Ok here are the other "tribes".
Bhakti Temple
Special Units: Tiger Claw (Replaces Swordsman)
Techs:
- Yoga (Melee units damage +5% more)
- Reinforced Gauntlets (Tiger Claws get +50% HP)
- Vegetarianism (Farms produce 40% more food)
Jesuit Temple
Special Units: Conquistador (Replaces Knight)
Techs:
- Flying Buttress (City +20% defenses)
- Smokeless Powder (Gunpowder Units have +30% attack)
- Christian School (+15% more research)
Shaolin Temple
Special Units: Rattan Shield (Replaces Swordsman)
Techs:
- Clenched Fists (Archers +25% vs Melee)
- Wood Clearing (Workers clear/chop forest 20% Faster)
- Dim Mark (Rattan Shield have 50% more damage)
Sufi Mosque
Special Units: War Elephant (Replaces War Elephant) [Needs a Better Unit]
Techs:
- Pilgrimage (Mosques cost 50% less to build)
- Fasting (Workers work 10% faster but -40% hit points)
- Sharia (Workers produce 50% faster)
Udasi Temple
Special Units: Chakram (Replaces Swordsman)
Techs:
- Sikh Gurus (Explorers get 50% more hit points and damage)
- Punjabi New Year (+10% more food from Rice Resource)
- Army of the Pure (Chakram can see 20% farther)
Zen Temple
Special Units: Sohei (Replaces Pikeman)
Techs:
- Zen Meditation (Meditation tech research cost 50% less)
- Master Lessons (Melee unit damage +10%)
- Meritocracy (Unit upgrades are 20% cheaper)
*phew* Ok so those were the rest I think.aybe not the best for a city state but perhaps a new type of map feature where diffrent regions can have their own far off temple in the middle or nowhere.
I think the bonuses need a lot of work but this gives something for us to work on and improve. I could totally see this done for other minor civilizations around the world.