I would rather ban Ponder and/or Preordain than Brainstorm. Those are the t1 plays, the real consistency cards. They enable the low land counts and the curve of cantrips. BS increases in power later in the game, which is inherently safer space in Legacy. BS feels powerful to cast and Ponder/Preordain feel like necessary paperwork to win. BS interacts with discard in interesting and mostly fair ways.