nitnit
One of two things will happen:
There will be a giant banlist
or
There will be far fewer OU pokemon than in Gen 4.
Both are extremely likely, but I think there's a 3rd, less likely option too:
A new metagame emerges where some previous ubers are no longer banned, and some previous OUs drop to BL/UU. This new metagame will either be reasonably balanced or really bad, and probably about the same size as gen 4 OU, maybe a little larger/smaller.
Of course, that's contingent on people not sucking really bad at tiering...