What often happens in alliances, especially in the real world, is that one alliance member often emerges to dominate the alliance - think of the Athenians in the Delian League, or America in NATO. So I propose more competition within alliances in the following way:


Alliance members should influence each other at double the rate which they would normally influence a non-alliance member. Some of the demands which can be made within an alliance should include forcing the member to remove an aggression/peace veto, as well as some mechanic to only allow for the dominant member to negotiate war and peace (with other members having have to ASK the alliance leader to declare war on/make peace with a third party. Although perhaps excessive, sanctioning (embargoeing) an alliance member could also lead to interesting mechanics. One other thing could be the following: there could be a "request to leave alliance" option. If the requesting member has more influence over the other members than they over him, he should be able to leave. If he has less influence, he should either have to pay a severe relationship/resource penalty or only be able to leave by declaring war. 


This would make alliance mechanics more interesting. The benefit of being in an alliance would still make members want to prefer to join one in spite of potential domination from within the alliance: protection from enemies, joint victory and greater commercial access. Perhaps to compensate for the above features, trade relations should be more lucrative between alliance members.


So either the leader is automatically determined based on influence without being obvious, or each alliance can require an alliance leader which alternates every certain amount of terms, with the one with the most influence being in charge.  This would allow for weaker membbers to eventually emerge as the dominant member.  This leader would have a live map view of alliance members' systems and, as said above, be the only one able to declare war or make peace. BUT this also means that the member states suffer no long term relationship penalties against the faction their alliance leader makes war on. In fact doing so could perhaps lead to alliance members being more inclined to leave the alliance - as they obviously wouldn't like to be used as pawns. As such the alliance leader still has to balance its overall power with some of the soft power of its members over him. 


But I do realise that what I propose above could lead to some balance issues. But I leave it to other forum members and the talented devs to at least think about this.