The first question that comes to mind is, how is this to be understood?
- Is a coalition alliance similar to a player alliance in which two entities decide to cooperate on some level, or ...
- ... is a coalition alliance more of a "coalition of coalitions?
I'm kind of assuming the first is the case, though the second would be much more interesting IMO, especially considering things like Alliance HQs and everything that could go with developing an Alliance capital.
If it is option 1, I would at least consider the possibility of alliances of more than two coalitions, perhaps even with a distinct name and logo to make them recognizable.
As to the possible benefits of coalition alliances, I suppose the sky's the limit (meaning anything available today in player alliances and capitals), though I would try to make sure that these benefits are somehow capped, so as not to put smaller, less well-connected coas at too much of a disadvantage. On the other hand, if done right, these benefits could be designed to favour smaller coalitions that are willing to join an alliance.
m2cw
P.