other then having the "top spot" or moving up in the credits ranking there isnt much of a goal of growing an alliance.
a few ideas i been tossing around
let members put coins towards storms my guys hate dropping 20 a piece on a storm outside the weekly freebie. Though if we could all pitch in towards one to get to 20 coins that would let guys feel more like a team then one guy dropping 20 coins.
or if you pay for the storm for your alliance you should be give a bonus of some sorts 20% more credits on any event jobs you do during the storm.
alliance wars or something where your alliance will go against another alliance for as many credits it can earn in a set time frame ie 24 hours now of course there would need to be some type of formula to give small groups the same advantage as large ones.. the winning alliance of course would get some sort of prize 5 coins a member or a extra free storm or something.