Manaphy will undoubtly create a dominate rain-based metagame, which I consider very harmful to the metagame. I don't believe enough has changed to make Garchomp OU; very few things have popped up to counter Yache Garchomp, the only viable new counter I know of is Jump's Trick Cresselia. OHKO moves will harm the metagame by making it much more luck based. Ho-oh, if you believe it should be tested, is far from OU without SR, so I rather test SR before Ho-oh, as well as all the above. If we don't have anything better to test than SR, let's test SR.
Don't speak so quickly, as you don't know how the metagame will change, do you? What if no one uses Manaphy because it can't get past Blissey, it's revenge killed by Shaymin-S and Swift Swimmers? What if no one uses the OHKO moves because they are unreliable?
To the people saying that the metagame has digressed to the point that Stealth Rock forces everyone to use rock resistant pokes, only about 10 pokemon in OU are resistant. On the flipside, 8 OU pokes are weak to it, half of which are in the top ten.