Trying for an anti-Thoughtseize that won't be a new Mental Misstep error. A free Outwit that must be fired before the opponent can try for the discard spell seems fair.

The trouble is, you just went down a card for nothing, and even if they do have the Thoughtseize, they're just going to hang onto it and play it turn 2 to strip away another one of your cards.

I'm assuming that by then you're not defenseless. You have mana to cast (or have cast, depending whether or not you think your opponent will go off-curve) Cursecatcher, Spell Pierce and/or Spell Snare. Or you may have Thoughtseized them yourself.

Anyway, the point is, you were defenseless until you played a land, but you no longer are now, so having delayed the spell is not nearly as problematic. I considered "if you haven't cast a spell yet" as an alternative condition, but didn't want something that was too likely to be a new mental misstep.

Really, the further details are probably a development thing if only because I myself don't play modern.