I've got this funny feeling that Mega Rayquaza and P-Groudon being allowed will be abused like heck - and that some Japanese kids will find a way past them, making the counter-Rayquaza-team the most popular all over Top 32.
A successful team in this metagame is one which can utilize its non-Uber-Pokémon in a meaningful way. Like, for instance, KO-ing or weakening the opposing Ubers to pave the way for their own Ubers. That, or supporting the Ubers to become even more fearsome. Follow Me/Rage Powder + Power Herb Xerneas springs to mind immediately. Either way, relying solely on the Ubers sounds like a plan that will only take you so far.