Also totally with you on this sort of design just being A++.I already run test subject. I'll probably be running both n ow. Might even cut a random 5+ drop for this. I love the shit out of designs like this, keep it coming Wizards. I can't believe how many cards in this set I'm already slotting in.
I'm not convinced this is a better card that test subject though. Subject wins topdeck wars and similarly super grindy games where this thing won't. Also, if you're holding a grip of counterspells and are even slightly behind on board, this might never flip where test subject would be inevitable. Test subject is also a pretty promo.
Good points about LTS. On the flip side, I guess, Thing in the Ice is a lot better in your opening hand, where you don't have to let it sit around for who knows how many turns. Jam it out then start casting your spells as normal. It's also a 0/4 instead of a 0/3, which is actually super relevant in low curve cubes like riptide's, because the number of less than 4 power creatures is quite high. I can definitely see the case for just running both like you sound inclined to do!
It's got me thinking about taking possibly wall of blossoms out, and shifting the 0/4-wall-with-upside to white and blue instead of white and green.