Kingdoms - Only have two (If you're bringing back origional kingdoms, then it would probably be Dustari and Zormite), keep it simple, this also solves the problem of other kingdoms "dying". The two kingdoms cannot be captured/destroyed (of course, attacking is okay) because, well, if they were, RPing would die pretty fast.
Forts - Turn Karakaze Caves, Ice Sarafan fort, Pirate Crew caverns and Samurai Castle into forts which can be captured in RP events, so that the balance of power among the kingdoms can change.
Of course, this only covers the fighting aspect of RPing.
Anyways, let the criticism and explaining of how this will never work begin
.