Defense pact

Uh, this would allow the creation of world-war/international crysis type scenarios. I like it. It would also be nice to have the option to create a unilateral pact (even with IP) where if only one of the the empires is attacked the other one automatically joins it; it would allow a better exploitment of "acts of strength" enriching the gameplay experience.
Personally I would also like to experience a better diplomatic "commerce" that would allow the creation of similar scenarios (ex: I ask a major empire to join a war to reclaim a territory that I recently lost. I declare that territory as a "war aim" in exchange of a something (a somme of gold/ the sign of a treaty/ another territory/ I buy for free one of his resources...): if, at war ended, my ally disagrees to give it to me I gain a grievance for that territory and he doesn't obtain the thing that I promised to give him; otherwise I would get that territory and my ally would get what I promised him). I would also like to have a "trade diplomacy" not just with empires but also among alliances (in particular when there's a conflict between a member of the alliance and the other ones)

I find it kind of puzzling that the existing Alliance mechanic doesn't really include a mutual defense pact. All that happens is your Alliance members incur a Grievance if they stay at peace with someone you're at war with, which seems pretty weak. And if you try to call them out on it, they get mad because you're making demands!

Although my only experience so far with an Alliance was with a Pacifist AI empire- maybe if they weren't pacifist they would have joined me in my defensive war.