It just occurred to me that the right comparison point for Rats might be Transmute. They've both got this "trash a card, gain another copy of the trasher" thing going on, but Rats has the advantage of being nonterminal, and Transmute has the advantage of being able to give you something other than just more Transmutes. (But since the most plentiful thing you usually want to trash is Copper, it's a useful comparison, I think.) Rats probably comes out looking better because being non-terminal is really useful, and because Transmute's awkward potion cost makes it poor fodder for other trashers, which seems like Rats' big selling point. But I think Transmute's the most similar existing card to Rats, so that might be a handy comparison point for how to think about it.
Transmute : Rats :: Remodel : Upgrade?