World War, as great as that would be, wouldn't work in hindsight. Kay made an interesting point in another thread, and it would be hard to have a real world war with two major separate RPing styles clashing constantly.
It would be easier to, yeah, have missions between the villages, though we'll kind of need GM's to help with that. Some random celebrations would be nice too. We've had celebrations before, but in order to have one to stimulate overall RP, we'd need to break down some barriers first, both literal and figuratively.
No world war is going to be breaking out anytime soon, not with the way things are going with all main villages allying and all that. So, how about a celebration in Jisiegakure, where we don't bring on the usual rules of engagement and RP with us? Auto-entry, random stuff, and all could even be allowed.
And since it's Jisiegakure, no major compromise will have to be made at this point regarding village securities other than maybe having your shinobi leave the village or whatever. Hosting the event could be the active GM's and/or Kages of the realm.