Gem from Dragon's Maze

This is my pick for the card with the most potential for Legacy/Classic:

Anything blue in a deck with Force of will is never a dead card.

That said, this card is a very powerful card for both its effects that are good in certain conditions even if each side costs 1 more than their conventional uses.

In Legacy, creatures are relatively plentiful for both sides to be useful.
It's good because you now get the flexibility to do what you want from a card that is in a slot that you conceded to for a defensive position. I had to think of using Diabolic Edict. Now I can safely run D. Edict on a taxed +1 mana for more flexibility and the Fusing part would be really breaking if it comes down to it.
It really gives you ways to get around those Show and Tell/ Reanimator decks.

In Classic, it's a better answer to Blightsteel Colossus.
In delver decks, I had a dilemma on what answer I could run to answer Blightsteel Colossus because once it gets down, the only way out is to resolve Jace, the Mind Sculptor and that is usually not possible (since Classic doesn't have p9). I once had the 1 mana bounce + 1 life loss spell in my testing so I'm glad that this card is around.

I look forward to playing this card in Legacy and maybe Classic.