This is a very simple mod. It allows all factions to confederate with any other faction. In fact it makes all factions having same blood and allows every faction to form a confederation. It could still have some issues and unwanted effects.
Steam link: http://steamcommunity.com/sharedfile.../?id=346644536
(with pictures)
NOTE: I made this because I wanted to allow you to "annex" other factions and I hope that anybody can help me with that. This modification is just a very simple way, but I hope you'll try this out.