Another suggestion for an MP campaign would be to ditch the standard victory conditions. These are designed for playing against the A.I., so they may be too ambitious for an MP game. Also, they more or less require you to go to war with your neighbours at some point. That'll hurt diplomacy right from the start - knowing that, eventually, it will be you or them.

As an alternative, faction specific missions could be used (e.g. "reunite Alexander's Empire" for the Ptolemeans and Seleucids, "unite the Celts" for Casse, Aedui and Averni, etc.). Or you could ignore the victory-conditions entirely and just play until an agreed year.