The natural comparison is Fold into AEther. Which was a counterspell with a "drawback" but was priced more expensive than Cancel because of the potential for abuse. This seems to make a huge stompy Eldrazi / reanimator-style deck way too easy. I guess it's just Elvish Piper with a restriction and requiring two colours... But is that a sensible design space to be revisiting?

It was meant to be a Master Transmuter variant, but I suppose she is in turn a sort of Elvish Piper variant.

Good point, Master Transmuter demonstrated that Wizards don't have a problem with Elvish Piper effects, at least as recently as Conflux...

I would also argue that, though this isa drastically different effect, it's far less annoying than the recently revealed Soulfire Grand Master.

Soulfire Master is certainly insane. Granting buyback to any instant or sorcery is pretty crazy. Definitely a mythic pushed for constructed to an irritating degree. But it is at least a tax of 4 on top of the card's normal mana cost - it doesn't bypass mana costs the way the cards we've been discussing here do.

That is true.