The reason I stopped playing MMOGs was that (1) these seemed to me to be mostly about the time you spend grinding and not about making good decisions and (2) the non-collision environment, i.e. you cannot really loose important assets (e.g., you die, you run to the next graveyard to respawn; after battle, all your units are available for the next one, as they are spawned from tokens, often represented as tcs [I mean really, virtual tcgs? Right baby, free yourself from the limitations of the table top games]). Well, maybe 1 and 2 are the same points, you cannot make bad decisions if you cannot make good ones and if nothing you do affects the game in a meaningful way anyways, well, then to some definitions it is not a game at all. Aaaaand I should stop ranting, as nothing of this would be an issue in a game designed by Dioxine...
As Rodulus said, coordinating geoscape and battlescape might be difficult - not only with two or more players engaged in one battlescape: As we know the resolution of a good battlescape can take RL hours and in a continuous geoscape your forces might be bound for in game days. This might be used by a clever opponent to raid your base, while most gals are away. This in turn, brings up the next problem, what happens when a player is engaged in two battlescapes at the same time?
Imho, xcom and all its mods suffer from the low speed and pace of the battlescape and this needs to be resolved if you want massive multiplayer.