I would like a checkbox option "[x] Allow in-game allying" that would enable people to ally each other during gameplay. How this would work; you'd press H, select the player from the list, and click a newly added "Propose alliance" button. The target player, and all the players who already are allied with him, would get a notice, after which they can either confirm, or not confirm the alliance. Every player of both alliances would have to confirm. Leaving an alliance should be as simple as pressing another newly added button in the H menu. A notification to all players would be given upon a player joining an alliance, or forming a new alliance with someone. If every player end up in a single alliance, the game should end.
Many RTS games already have in-game alliances, one of the ones I've tried being C&C Tiberian Sun, and I think it allows for much more interesting matches than without, especially when it's FFA. Sure, some people, who already have the upper hand, might gang up on weaker teams, but at least it doesn't happen so often in TS. I think the Spring community has fairly enough honor to play fair.
One more thing... I don't know if it's already possible, but I'd like to have some button to show a list of alliances in-game. It's just something that has been missing all the time, and it'd be quite useful especially if in-game alliances will get implemented.