Ok, hear me out on this one. Maybe this idea is a little far fetched, but maybe not.
A neat feature would be an "alliance" feature, whereby, in a non-team game, 2 players, can agree to "ally" for 1 turn, and 1 turn only, in order to fortify the other's army.
Player A and Player B agree that they are going to join up to wipe out Players C, D, and E. Player A would be able to select some kind of option that asks Player B if he accepts Player A's request to have a 1-turn alliance. Playber B would have to accept Player A's request in order for it to be annouced automatically in the game chat. Then, at the end of Player A's turn, he would be required, if physically possible, to make 1 fortification (either adjacent or chained, depending in the game option, and if the option is unlimited, it would be 1 chained) to an opponents territory. If Player A wasn't physically able to do so, the alliance would be negated at that time. When Player B's turn came up, Player B would be required to make a fortification to Player A before he could completely finish his turn. If Player B is not able to do this, then the alliance would be negated at that time. Once both players have completed 1 turn each, the "alliance" feature would be disabled, and would need to be re-initated by a player again. So, the "alliance" is only allowed for 1 turn.
The reason I thought of this was that I was in a 6-player terminator game, and another player and I formed a legal, announced, alliance, however, the other player had a few territories in a region I was trying to conquer, and he wasn't focused on that region, and I thought it would be really neat if he could fortify my armies for one turn, in exchange for me fortifying him in another location on my next turn - simiilar to what you'd do in a normal team game. It would only be for one turn, b/c it wasn't a team game to begin with, but the two of us could further strengthen our alliance by being able to do that just once if we both were able to do this.