Cool thread. I feel like rares in particular are in a bad spot as far as cube design goes because the rare cube space is huge.
Last weekend I spent time reading through the "this or that" thread on MTGS for peasant, and the discussion is very constructive by comparison to the rare version of that same thread. In the rare version, all you generally see is X > Y with minimal or no syntax. It's just a power question. Not true in peasant, where more than one person specifically indicated that they didn't run cards for power reasons only (and said it speaking from a community perspective). So it's more about context, something I feel is 10 times more useful.
I bring this up because I think most of the playable common and uncommons are being actively discussed due to this different mindset - nothing is really rejected outright on grounds of raw power. Not that there aren't some cards dropping below people's radar, but it's the exception not the rule. In the land of rares though, I could probably come up with 100 cards that were either under tested in rare lists or completely skipped over which would easily be cubable at peasant level for sure and at lower rare power levels as well. Some cards are even good enough for high powered lists too if people took the time to actually play them (
Meren of Clan Nel Toth), but there's only so much room even in the most open minded of lists.
Anyway, here's a card I added awhile back that got 30 seconds in the spotlight before being immediately replaced by whatever got printed after with more raw power, but this card has proven to be a fun card with build around potential. And I sort of feel like it does the demon drawback thing really well too, so hits a flavor home run for me as well.

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This has appeared in several types of decks. Control decks with things that can be sacrificed to it later in the game. Graveyard centric decks obviously (where you can make the drawback immaterial). Even as a top end card in some aggressive decks (tossing one drops to it after they can no longer do anything). Token decks. Recursive threat decks (crawler, et all). It has immediate board impact, and while it's an expensive doom blade + fog effect if that is all you get out of it, it's got a lot more to offer if you put even a minimal amount of effort into abusing it. And it's a scary ass threat too being a 5/6 flyer.