In the absence of an OFU, whoever first writes a mission to the 'verse and encounters minis gets to decide what the minis are. (It's polite to have a discussion about it first, though.)

I'd be inclined to use mini-White Walkers, personally, since they pretty much threaten everyone equally. Or, since humanoid minis creep some people out (though I dunno if that necessarily applies to clearly monstrous humanoids), maybe mini-winters. They'd be like little mobile blizzards. Maybe with glowing blue eyes set in the clouds. That might be too cartoony, but I kinda like the idea of OFU students retreating through the halls shouting "Mini-winter is coming! Mini-winter is coming!"

Alas, GRRM is unlikely to change his mind about fanfic anytime soon, so this must remain a hypothetical discussion.

~Neshomeh