Need more rewards: Trading ships (i.e. TEC player gives Advent player a Marza, 10 Hoshikos, and a carrier, at the expense of the Advent fleet points or TEC fleet points, depending...); Trading tech (Armor/HP/Shield...) (These can't be taken back... one can select what techs to give); Share structures (like Phase Gates and repair); Share fleet points (one player runs econ, the other military. The econ guy gifts the fleet points to the military guy who does with it what he wants. Can include both race's ships in the fleet, it's just all under one person's control. Although I'd be the only one to do this...)
culture bonuses affect multiple empires (so 3 Advent players allied = 18% mitigation, 3 Vasari = 45% damage... maybe not just a straight stack, but you get the idea. This only takes place where all three culture areas overlap.); Culture can meld in with other culture, not just shove it around.
Not sure about the cease fire... perhaps if the time to complete mission was shortened drastically (halved?), and the time to offer mission was kept the same as it is now.
How about "Help Me" missions? especially easy to complete as Vasari with phase gates all over the place (Kotsura!)