Searching for mods (looking for mods for Tribes of the East).

That's not quite what I meant; I'm talking about having the option to customize game modes when selecting a map in the game. There were even more complexities and intelligent AI, meaning I could choose any existing map and play with friends in a cooperative mode against the computer.

Then, it seems you're talking about Quantomas AI (also seen under the name Eternal Essence). Game modes and increased difficulty – that's what it offers. I don't remember if cooperative modes are included "out of the box."
Some of the described functionality is available in Might & Magic: Heroes V. Check their website or directly in the game.
Link to a potentially useful mod:
Quantomas website