I feel as if this metagame is really going to revolve around rare, powerful boosting moves. Quiver Dance, Shell Smash, Dragon Dance, Shift Gear, and Belly Drum are what I'm looking at right now. Every single Bug, Normal, Steel, and Dragon-type are now going to be receiving a massively powerful setup move that turns them all into monsters in one or two turns, flat. Things like Garchomp and Kyurem-B get Dragon Dance now- seriously, imagine Dragon Dance Kyu-B. Bug-types all get Quiver Dance now- and as the OP stated, Yanmega is going to be a big threat. Steel-types get Shift Gear, which is awesome because it alleviates their offensive weaknesses in one shot- by doubling their usually low speed and increasing their usually moderate attack stat. Normal-types get the best of it all- Shell Smash and Belly Drum are two insanely good stat boosting moves, and with powerful STABs such as Return or Extremespeed, Normal-types that carry Fighting-type coverage will be able to obliterate most of the tier in a few seconds.
I really can't wait to test this out. Nice idea!