Now there's an interesting question: should this ability be black, or mostly w/r?

Black usually exiles only things that are already in the graveyard. I'm curious if it'll keep getting straight-up exiles like Gild, Silence the Believers and Macar. Note that no such spells were in M15 or Tarkir Block

I'm not sure the difference from Carbonize is worth it.

I think black is still allowed to exile, like red and white (and blue when it polymorphs things). But the default should be to destroy. King Macar exiles I think because the flavour is he's turning them into the Gold token, so it's the same reason that Curse of the Swine exiles its targets: because reanimating things that have been shape-changed feels wrong. Gild likewise, and Sever the Bloodline I think is for similar reasons. Silence the Believers I don't know - perhaps to alleviate confusion about Bestow creatures?

It was just an intriguing idea. Generally black prefers mucking with stuff (including exile) once it's in the graveyard rather than upfront. Silence is clearly your classic third-set safety valve against bestow (see also Deicide and Back to Nature re: enchantments in general).

Although these various cards have specific circumstances, they are still unusual and oddly concentrated in time, hence my wondering if we'll stop seeing them for a while.