I think this ideas will not improve team play a lot. Team play always exist between active players, so the problem is not the mechanics of the game but to make player active and not bored.
1. Make the mid game more important, putting more action.
a. Attack merge: You invite a friend to make one attack together and the attack will hit like one. Will be easy to hit enemies treasure and steal VPs. (not allowed for same ip/macs)
b. Minor objectives: How about release minor artifacts in Natars that works for 1 day (maybe alliance effect). Burn troop to get artifacts or use then to hit enemies...
c. Adopt a player: Players can adopt one other player with pop with 20% pop or less than you to give him production bonus while in the pop 20% condition. This will make players that start late be more relevant.
d. Make every target more important, not only few villages. Ex: Treasures can hold only 1k but there are 4x more treasures actives. This can split the action.
2. Tech trees can be nice, but not sharing blah blah blah Just give us more options to customize our game play.
a. Offensive | Deffensive | Spies | CP generation | Resources | Merchants | Buildings
Just some ideas.