Yo, finally: I GOT ideas, don't know if anyone else already brought them up, but hey - here they are:
1. Already in the Main Topic: economical use of surroundings: big forests for lumber, mines and quarrys in mountains and different conditions for agraculture - f.e. breeding goats and horses in stepps, growing wine and making cheese in mountains, growing southern fruits in oasis of deserts, breeding reindeer in tundras
2. Already in the Main Topic: New-World crops and recourses - guinea-pigs, paprika, cacao, tropic lumber, large quantities of certain gems (emeralds)
3. Already in the Main Topic: customizable defences
4. Fortifiying camps, while resting
5. Customizable baggage trains - large quantities of food AND ammunition supplies
6. Weather and Seasons
7. boiling water and oil for defences ( oil is expensive and takes longer to heat, but can be set on fire)
8. lime bombs ( I live near the "Harburg", where they had tons of that stuff, making it impossible to storm the castle while the wind was blowing against the attacker, but could also lead to a catastrophe for the defenders, if the wind turned)
9. penalties in tundras, steppes, jungles and mountains
10. tactical usage of fire
11. naval artillery ( in this period: catapults, or a FEW cannons on the front of the ship)
12. several kinds of sea raiders
13. more desert and new world factions
14. Jumne and Balion ( Balion could still be brought into the game without having to change the new world by making it an island - nation)
15. Rationing the supplies (quantity and types of food)
16. Aztec - Religion for Acztaocs
17. More variety in quests
18. Option to learn Acztaoc - language
19. learning troopformations like poems and bringing them into the army
20. And since I'm done for now ... one thing typical for the mongols and therefore for the Khergits: YOGURT!