Could it be possible to limit it to one nation/one Alliance, not be the leader of one Alliance, but join another without resigning from previous one??
(Example, only member of Brotherhood of Nod has joined another Alliance)

I am thinking that there can be a "need" to be in different alliances. Just because one helps you on this continent, doesn't mean it will on the next.

i like the multiple alliance possability. It makes for more complex diplomatic situations.

You're thinking of MDPs? I think the alliance thing should be split, having multiple alliances/allies but being part of only 1 coalition. Like the alliance system right now is more of like a Coalition of nations willing to help and support each other. Whereas an alliance between nations would be more personal of 1 nation helping another.

Peace treaties would become peace treaties, alliances would be like an extension of that between single nations, and a coalition would be a group of nations allied to each other with a specific cause/group.

I really like being able to have different alliances. I'm getting ready to start another one for my eastward expansion.