BEFORE YOU WRITE YOUR SUGGESTIONS PLEASE READ HOW MODDING WORKS IN THIS GAME
HOW TO MOD THIS GAME?
You can't. Well, you can, but not on an official level. There are no real tools for modding (except for a few utilities that make modding the files a little easier, but also tend to screw them up) so the whole thing is done by editing the .xml files which contain various values (from hitpoints to damage). However that represents only ~30% of the game. The other 70% of the game is hardcoded (mostly game mechanics).
That being said, the files don't contain everything you would want when you want to mod something in the game as the list of available options is pretty short. You can edit stuff like damage, cost, damage type, etc. but if you want to, for example, make a specific unit garrison in a specific building, well, good luck - there is no option for that.
On top of that the game has a predetermined (total) number of all entries in the game (which cover everything from buildings to nations) and if it detects that there so much as 1 entry more or 1 entry less in that total number, the game just crashes. So in other words: if you add something or remove something in this game it throws a hissy fit.
HOW I MANAGED TO MOD THE GAME
So you're now probably wondering how I managed to add so much stuff without getting this error? Well I basically used the entries that were already in the files but were disabled by default (such as CTW exclusive units or houses) and used those entries for the new units, buildings and nation powers. As for the latter, there were also a bunch of nation powers that were disabled by default (the reason being, some of them didn't work properly or weren't fully fleshed out, but some worked fine). I also exchanged some units and used their entries for new ones. And recently, I've been running out of "fresh" entries to add new units to, so the remaining amount of new units will probably replace some placeholder ones and not add much new (also because of balancing purposes).
Some other methods I've used (such as implementing naval generals) was to trick the game into thinking a unit is something else. For instance: a naval general is treated as a normal general, but I've changed the model and classified it as a ship. I've used some other methods to trick the game into doing what I want, but those are just a few.
ABOUT THE AI (SINGLE PLAYER MATCH)
The AI is still vanilla, which means it treats the mod as the vanilla game. Since the mod makes extensive changes to many different gameplay elements, the AI does not utilize the new system very well. It can create a very basic economy and advance in ages more slowly than the player but it will not utilize all the new units and buildings properly (for instance, it will use Horse Raiders as just another cavalry unit in its army). I've been planning to update the AI for a while now, but a lot of its coding is hardcoded and the only way to break that is with scripts (which tend to desync in multiplayer), and even those do not give me all the AI customization that I would want. So for the moment, if you're playing against the AI, play on Tougher or Toughest, so it gets a resource boost and can actually compete with you.
WHAT CAN AND CAN'T BE DONE
So what can realistically be implemented in the game? Well, considering all of the above problems, a lot of it is determined by trial-and-error. I've had to scrap TONS of ideas for units and buildings because of this.
Here's just a short list of what can't be modded into the game:
- garrisonable structures (by default you can only garrison a unit into the building that produced it, except for the ones already in vanilla)
- adding game mechanics (pretty much impossible as it's all hardcoded)
- adding nation powers (all of the powers are tied to nations and there's no way to edit what they do)
- adding ages or nations (except by changing the existing ones)
- adding wonders (these are especially bitchy when you start modifying them)