­Elvish Mystic + Questioner Imrahim

So, bog standard Llaanowar Elf variant, and.. unsummon a creature to decrease the cost of a spell. Most obviously used as a flicker; but can also polymorph. Nice.

So, we're making the mana - and we're doing it in as the unused colours.

Black typically makes mana through sacrifice, and makes it in bursts (though that's now red's territory; there's still a few altars around).

White... doesn't usually make mana. It washes mana. In ye olden dayes it paid culmulative upkeep because snow is white. And there's Sunseed Nurturer. That's about it.

White does get "Save my creatures" a lot, the other half of Imrahim.

So, let's do that. Pretend this card was a contemporary of Personal Incarnation and so lived back when there was mana burn. (I'll even word it more like it would have been worded then, to help you - nowadays there'd be "ueot" and "add that much to your mana pool" in it)

Back in those days, it's really rather fair. It's "Regenerate target creature" which is good - but at a cost of "Oh, right, this means that I take that damage instead" unless you can find a sink for the mana; probably mid-combat. So that captures the black bit, of being lifelossy.

Nowadays... when it drains away harmlessly? Eh, it's still probably fair, though not NWO-safe, so make it rare. After all it just saves a creature and you can see it coming.

I guess I could make it only protect itself; but then, well, the incarnation already exists. This avatarises other stuff.

Pretty nifty for chaining multiple Earthquakes / Blasphemous Acts etc in the same turn.

You're pretty much spot on with your implied modern wording: I'd go for "Until end of turn, if damage would be dealt to target creature, you may instead prevent that damage and add that much white mana to your mana pool."