The matter of Coalition Capitals, attack limits on them and the cap level has been brought up in the past. A couple of months ago I have already sent player feedback on the matter to our dev team, that is as much as can be done at the moment. We feel that diplomacy and teamwork are imperative to gameplay in Sparta and by making allies you can deter larger or stronger Coalitions from attacking their smaller counterparts. 

Changes in the future are not entirely impossible, but at the moment I have no information from the devs about anything being planned in this regard.