From playing in the Dream World tier sometimes, the metagame revolves around Genesect. Scarf Genesect is one of the best answers to Latios and Latias. Its like RBY Chansey; first one to lose their Chansey loses the game. The same applies with Genesect. Its a solid special attacker with Life Orb, but its even a better Scarf user. Genesect may go to Uber tier, but once Shadow Tag Chandelure comes out, it'll come back to OU. Genesect's effect on the metagame will be very large.
Tier: OU
Meloetta is really lackluster. Step Form is really the only good thing about it, but its easy to intimidate, thus causing it needing to transform again.
Tier: BL
Keldeo, my favorite out of these guys. Its very scary in rain, as it boasts the strongest Hydro Pump in the tier. Its also very fast. Keldeo is also one of the best answers to stall available, with access to Taunt and Secret Sword. Its a phenomenal "mixed" sweeper, and is something that all teams need to prepare for. Its only countered by Latias, Latios, Jellicent and Celebi.
Tier: OU
Other cool abilities that will have a large effect on the metagame are: Technician Breloom, Imposter Ditto and Shadow Tag Chandelure. When I play in the teamworld tier, Excadrill is legal. Technician Breloom is the best counter to the mole, as it kills it with Mach Punch. Technician Breloom is only really countered by Dragonite, Latias and anything else that resists both of its STABs. Imposter Ditto is the best revenge killer around when it has a Choice Scarf. Shadow Tag Chandelure is like a better Dugtrio.