{thing in the ice} with one ice counter left. I cast {transgress the mind}. Thing transforms, returns all creatures to owners hands, one of which was oppenent's {etched champion}. Transgress resolves, and I choose the champion. Legal?
100%. Transgress was on the stack when it triggered Thing, Thing goes through all the motions and bounces all of the dudes, we go back to Transgress which is again on top of the stack, it resolves and you look at your opponents hand. That's pretty slick!
Totally legit, since you only pick the card on resolution. Say your opponent had {Aegis of the Gods} out, you wouldn't have been able to cast Transgress, since you'd have to pick a legal target when casting the spell, even though it would be a legal target by the time Transgress'd resolve.