An example of my "use creatures in various ways as costs" suggestion.

(I also considered "Target creature gets -X/-Y where X/Y is the tapped creature's power and toughness", but decided I should start with the simplest possible example.)

Hmm, doesn't work so well for the "Slave race" flavour, because, well, high power creatures. Which means agressive attacky ones.

Yeah, it was inspired by someone's comment that they envisaged noble races enslaved and I thought "what if the enslaved were not all 1/1". I'm not sure that's right, the flavour isn't quite right, but I thought it was worth exploring because (a) using bigger-than-1/1 creatures as sacrifce fodder is a bit new mechanically and (b) it feels more wanton/decadent to enslave something great and noble than a bunch of 1/1 tokens :) (I assume we will still have plenty of use for sacrificing cheap things.)

This particular card could be tweaked to be toughness or CMC: the question is, do people think "costs that scale with the creature" is interesting enough and flavoured enough to think about? (And if not, is "lots of sacrifice" enough to build a colour identify round.)

This seems like a nice occasional mechanic. Reminds me of Induce Despair, but you have to have the creature on the board first.

Yeah, very similar. I knew there were lots of cards like (in fact, it seemed likely this card existed exactly, but I couldn't check because gatherer was down). In fact, I'm surprised Induce Despair costs as much as it does: -X/-X seems comparable to "destroy target non-black", and the extra card seems a notable hurdle (less so than having a creature in play, perhaps).

Way to remind Prey Upon that green sucks at removal. :P

Hey, green? You suck at removal. You really really do.

Uh, why is your 8/8 stompy thing still 5/5? And why is it looking at my tapped 3/3 like that? That's the biggest creature in my deck!