I've seen this proposed before, and I believe that the problem with that idea is that there just aren't enough mons to really justify this. Assuming that the tier consisted of any Uber that was banned from OU, you'd pretty much end up with an overcentralized version of OU. Now, the tier would either have to use the Ubers mindset of "Battle of the Best of the Best" or the OU mindset of trying to have a non-centralized metagame. If you use the OU mindset, then you're just going to end up with another OU.
If you use the Ubers mindset, then you're dealing with a tier whose boss mons are pretty limited compared to Ubers: MegaMence, MegaBlaze, MegaGar, MegaKhan, MegaLuke, MegaMaw, Genesect, Aegislash, Greninja, and Deos S, D and N. This seems like a lot, but in terms of power not all of the megas are really equal, so you would most likely only see Mega Mence, MegaGar, and MegaKhan for the most part, thus leaving you with a small pool of new toys that aren't already usable in OU. If you try and take out the quick-banned ones to diversify the number of Megas used, then you're dealing with a very small pool of boss monsters. You'd basically end up with a tier whose player base consists of players who don't like Ubers, don't care about overcentralization, and want to use the mons we banned in OU in conjunction with OU mons, which is not a large enough player base to really justify this.