Troberg brought this idea in another thread:
http://ufoai.ninex.info/forum/index.php?topic=2270.0. The obvious problem was how to handle the flow of time. So here's my idea:
In geoscape mode, any players that have permission to do so may change the flow of time, unless anybody is in battlescape mode. Permissions are given by the server's owner.
While anybody is in the battlescape, time still flows, but is fixed at the slowest rate.
When entering battlescape mode there's an additional option: Wait. This would wait until another player's craft is in position.
If the map supports it, multiple players can cooperate for the same mission. If it doesn't, the game gives some excuse (terrain too rough, only N craft can land), and the players decide who does it. In cooperative missions, players land in different areas of the map. Loot is split equally among all participants.
Players can simultaneously work on different battlescape missions. The "wait" option would allow everybody to get their craft into position.
In order to keep players from getting bored in geoscape when somebody is taking ages in battlescape, offer a "simulator" option that allows to play a simulated game against the computer that can be aborted at any time.
This is as far as basic mechanics goes. Something that would be nice is having cooperation be optional. Having two organizations compete and even fight each other could be very interesting.