It would be nice if R&D would be a little more careful with printing cards with serious implications for Legacy. Mechanically, Snapcaster Mage ought to be red, and printing a flying blue Wild Nacatl was kind of silly. (And we know what Mental Misstep did to the format.) These cards are interesting in Standard, but because Development doesn't test Eternal formats much, we have to live with some poor decisions.

There are a lot of interesting options for cards that hose Brainstorm. Maybe a Chains of Mephistopheles type of effect on a red creature? A black instant that says, "Until the end of the turn, if a player would draw a card other than their first card drawn in their draw step, instead he or she does not."? There are a lot of options.

One issue is that it's tricky to print things to hose blue that aren't good in blue decks. (You don't want your hoser card to just be tech for the mirror.)