Repeating the same point that has been said many times by now, but is nevertheless worth repeating:
It is a great idea, especially since a considerable (albeit minority!) part of the playerbase would love to see some UU mons get the chance to shine, spicing up the game.
At the same time, I am opposed to enforcing it the way you suggest, where everyone would have to play UU for a season and then switch back to OU.
For one, forcing players who most likely had to spend a lot of time acquiring a solid OU team (hunting, buying ccs, w/e) to build an entirely new team will put a lot of players off the game.
Secondly, only having a UU meta for one season at a time will not allow for a stable meta to be formed. By the time the meta becomes legible, we will have moved on to OU again.
The reason why a lot of PRO players complain about the meta being stale (although we can argue it has been the "freshest" it has ever been and will change again with the new megas) is because the MMO aspect hampers creativity. If you have to grind and put in the hours to get a playable pokemon, you will try to go for the mons that gives you the most chances of a win. This was obvious a few seasons ago when we had the same boring core of conk + tank chomp + weavile etc being spammed to death. When playing Pokemon Showdown (and I know we hate this comparison, but it is a valid one) you are more encouraged to experiment, since you are not punished for it. You load up a mon, give it a try, if it fails, that's the end of that. In PRO this would mean hours of grinding and it would end in frustration if it does not work. All this to say: I think having a stale meta comes with the nature of the game. More than in any game, people want to win especially since you get tangible rewards for winning (coins, mounts, guild island etc.) I am not sure whether introducing a forced UU season fixes that issue, it just serves as a distraction from it.
I'm not saying that PRO needs to change, we play PRO because it is PRO. Perhaps I am more so saying that a stale meta is, in part, to be expected. A separate queueing system for UU would be great, however, giving those who have the time and money to hunt UU mons an alternative to playing the same core over and over again in OU.
Tldr: Sounds great, in practice probably not the best idea.