As already mentioned, Sleep and Species clauses are a must. So many teams in Generation 4 had trouble with absorbing 1 Sleep user let alone facing the possibility of everything sleeping. Without Species clause, a chaotic metagame could become even more difficult. There are tons of new Pokemon to be prepared for, and facing multiple Pokemon of one type (like 5 Kingdra in rain, etc) could become very overwhelming.
I'm perfectly fine with testing Evasion and OHKO clauses. They are already luck based strategies, and most top players aren't likely to employ them. That being said, I don't foresee either making a huge impact on the metagame regardless of whether we choose to test them or not.