What would be nice is if you could fortify and assign troops to your allies in games that aren't doubles. That way, if you have an informal alliance, you could reinforce the "enemy of your enemy" as a counterpoint to some one else.
That would make cheaters more powerful, and honestly, I don't think it will be as effective and probably lead to more mistakes than interesting strategies.
Sometimes when I'm playing games, usually Assassin or Terminator, Someone about to be killed, and I wish I could fortify my armies into their territorries. I think this would be an interesting addition to the game, at least in Assassin where its every man for himself. THe only drawback to being able to Fortify into other players is that in Standard and Terminator games, alliances will be much much stronger. (And much much cheaper...)
Iainarm, son of the great Iainleg, son of the great Iainhand, son of the great Iainfoot...
A) I don't see the point unless your allied with them, in which case that this falls under the already suggested alliance selection
B) It'd be ripe for abuse by multis and secret alliances, also fortifying people because they're being slammed on would only encourage others to accuse you of having a secret alliance whether you are or not.
It's impractical for many reasons, probably more than the two above.
Initiate discovery! Fire the Machines! Throw the switch Igor! THROW THE F***ING SWITCH!