How would you think the nations would change if our OCs took leadership of them? Let's just say in this scenario:
Yasu-Kazekage
Taiyo-Hokage
Kai-Mizukage
Tomiko-Tsuchikage
Kenji-Raikage
Hmmm,
The Mist ceases to be one of the Five Great Nations, it ends up more as a lone nation because of the Yuki Clan's brand of ruling. It's still a force to be reckoned with, but it'll never be able to stand on-par with the other 4 Great Nations again.
With Yasu and Tomiko at the helms of their respective nations, a gradual peace is restored and Suna and Iwa form a shakey, but somewhat feasible alliance. Konoha being Suna's ally, and headed by Taiyo, also join in with this alliance. The strength of the three together is completely unstoppable, they'd conquer the world overnight if they so desired, but that'd never happen. They're all tree huggers, so they suggest diplomatic relationships. Kenji is of course, perfectly fine with this. The already unstoppable trio becomes a peerless quartet. Their power is so great there isn't another force on the planet capable of being on par with them. With all their leaders practically sleeping in bed with each other, the four nations swiftly dominate the continent, they act like the UN and grow stronger in every aspect by mutual reinforcement.
The Mist receives some benefits, but it'll never compare to the likes of the improvement going on in the quartet. There isn't quite free movement because there are nations between the four powers, but relations between them is about as good as it's gonna get. Of course, peace is ubiquitous, there's simply no reason for anyone to go to war. The four nations don't need to fight each other, and there literally isn't another power in existence with the balls to defy them, or the strength to actually make that defiance a reality.
This unparalleled prosperity continues for a while, but something inevitably happens, a spark is lightened somewhere, and prosperity is often mere kindle to the flames of war. Maybe the Shinobi with no need for their services turn to banditry to survive, but that isn't too bad, after all, what forces do remain in the United Shinobi states can easily handle however many rogues pop up. Maybe the lucky ones can turn to industry, increasing production to a never-before-heard-of scale.
However, regardless of how well they prosper, it'll never last. All things follow the cycle of samsara, and even the mightest of alliances will wither and die. Maybe it's a deep-seated hatred between Suna and Iwa, that couldn't quite be wiped out so quickly, or maybe new social issues and divides created by the new class of working-shinobi who the larger majority- the commoners- could never compare to in terms of production capability, maybe it's just the fact that Yasu's in bed with Tomiko and some extremist Iwa shinobi, bred to have a Stone-will -originally referring to their determination to carry out orders to the death- can't stand the idea of Suna scum defiling Lady Tsuchikage and tries to assassinate one of them, or better yet their child, whatever the reason is, there's public outrage which can go all sorts of directions, especially if it succeeds *see WW I*
The fact of the matter is, divides between Ideologies are the hardest to heal, and however small they might seem, are enough to rip even the greatest of empires apart. They might even be able to hold things while they're still alive, but after 3 or 4 generations, children brought up in peace know not the brutality of war, and competent successors always seem to be lacking in times of need. Just like the Great Empires before them, the Chinese Qin Empire, the Roman Empire, the Byzantine, the Persian Empire, the Sassanid Empire, the Might Mongols... eventually they fall to internal turmoil and fracture, as all do, with the passing of time.
It's a beautiful period of history, but history is just that, history.
The Mist may or may not have imploded before then, but even they don't survive that much longer either.