^I'd like to disagree here.
It would not destroy the format if it was made in a balanced way. Just as Machamp didn't destroy SPs, even with its OHKO for F and the old Rare Candy rule, as well as BTS, a Stage or even a Stage 1 that could Take Out would not be so bad for the game. By playing it you would risk not playing against Reshiram or Zekrom, and chances are even if you did, it would become a prize trade.
Kingdra does not wreck Reshiram. It does 40 damage to it, maybe 50 if that's what you use its Poké-Power for. There's no way you can tech Kingdra for Reshiram matchups; it doesn't make any sense.
Donphan USED to wreck Zekrom. Does it now? No. They have Tornadus. Phanpy can be Catchered now, too, when it hits the field. And "teching" a Stage 1 that requires a specific type of energy is often hard to do in many decks, anyways: devoting 4 spots to the Pokémon as well as about 4 for energy (or a switch to Rainbow, which isn't very desirable) is a huge deal, especially when the opponent will simply 2HKO it with Tornadus.
A: Some decks simply can't be prepared for Reshiram or Zekrom. Stage 1s will fairly consistently lose to ReshiPhlosion; there's not really a tech that would make that better. Same goes for MewLock and Zekrom: MewLock, even though it has room for a toolbox, is simply not good enough to play Zekrom and win a large amount of the time. If you tell me a tech that makes the matchup better, feel free, but there's no way to make any deck Reshiram- or Zekrom-proof.
B: Even meta decks will lose against Reshiram and Zekrom. I guess you could play Gothitelle or MagneBoar for good Dragon matchups, but those leave susceptibility elsewhere in the meta. In addition, no format should force you to play deck X to win against deck Y, which is a large part of the meta. That's just unhealthy for the game; it doesn't let people choose decks they like playing and it focuses more on counters and matchups than on the skill of a player.
C: So you're bad if you want to play a deck that loses against some OP Basics? All decks have bad matchups, yes, but these are cards that can't be teched against without changing the entire focus of your deck, and that is the overlying problem. MegaZone can tech against Stage 1s with Jirachi. Against Gothitelle, Zekrom or Reshiram can play Magby. There are low-card techs that make matchups better in this meta, which is good. But there is nothing that could be called a hard counter for ZPST or Reshiphlosion here, and that is the ultimate problem.
This single card would not destroy half of the metagame, just as Machamp (as I mentioned earlier) did not even with the old Rare Candy rule, BTS, and no Catcher, as well as a meta largely comprised of decks entirely focused on Basics in the format. The only thing we still have is the last one, which is what would make Take Out useful. It would not be OP, simply balancing. It would diversify the format: give decks that lose against Reshiram and Zekrom a chance in the format and lower the amount of people playing cheap (in cost and probably effect), overpowered Basics, opening the format to decks more based upon evolution, which nearly always leads to a healthy format (because don't people always complain when there are Basic Pokémon in-format?), as we see in former sets. There's so much more strategy and diversity.
Say what you will, but Take Out would be good for this metagame.