Yeah, I definitely need to look for some more rules/mechanics to encourage warfare.
Maybe disallow player alliances and set up a proper team system rather than standard alliance blocs:
Blocs are formed by vassalization, which is permanent, but nobody is allowed to agree to peaceful vassalizing, and losing x amount of wars/war percentage means the losing side has to accept vassalization. Then have a hall of fame type thing and restart play every few months, with leaders and players in the winning bloc both recorded.
That would encourage more CBs being gathered so that crucial wars can be intervened in, on whichever side, and also give defeated players something to work towards.
There would likely be issues with balancing, but having a malus to starting nation for previous winners might sort that out.
Possibly allow the leader of the winners to assign rankings to their followers in the hall of fame, too, so that the difference between a player who really helps out and one that just tags along is actually recorded.
Maybe have a 1 or 2 session period where player vassalization is not allowed, to let people grow a bit or something - though that could be really dangerous, like what happened in the last EU3 game with england.
Just some thoughts anyway