A Phoenix planeswalker. She could pretty much be mono-red, except that that last ability seems so white to me.

The Phoenix flavour probably lets that last ability (and thus the whole card) be mono-red. I love it.

I'm glad you like it. Her ultimate is powerful but not impossible to work around. It does lead to some infinite combos, but maybe that's okay for a planeswalker ultimate.
I wonder if a phoenix can even hold a spark. I suppose it depend upon whether or not it was sentient.

Nice tension between the first two abilities too.

The ultimate is very nice; comparable in power to "Enchanted creature has indestructible" (which I guess is where your white comes from) - but Darksteel Plate says that can be splashed at not tooo much cost. (Also; ugh. Why can't it be "Is indestructible"? So much templhate.)

It does go ridiculously infinite. My first search turns up Accursed Centaur though that's not actually usefully infinite. Aerie Ouphes will only clear out flyers; but Alchemist's Apprentice means I don't think I need to look any farther.

Still; a difficult planeswalker ultimate that allows you to combo with your other stuff for huge amount of gain? I don't see a problem with that (other than my usual planeswalker hate, anyway).

And the ability to name a card before your opponent knows you're using it? So they KNOW what's going to happen; but it's too late now, shoulda killed the 'nix when they had a chance? Gorgeous.

(I kept looking - Bloodflow Connoisseur comes sooo close to working. If only it was a modular vampire. Bloodpyre Elemental kinda smacks of an "I win" button, though. And Boggart Forager is hilariously annoying. And many of the good cards to combine this with are red? Wonderful!)

It gives you infinite mana with Ashnod's Altar, though I think Legacy and Vintage can handle it.

Infinite mana in Modern with Thermopod, but again, that's fine given this is an ultimate. Johnny could have fun with a deck stuffed full of things that go crazy with the ultimate here. (In fact, this is a great planeswalker to use Rings of Brighthearth with: that'd let you create two emblems for firing off the ultimate!)

V: It actually used to be "is indestructible" - see Darksteel Pendant etc. That led to some weirdnesses, though, such as Snakeform removing indestructibility in some circumstances but not others. Now that indestructible is a keyword ability like flying and the rest, those rules interactions are much more consistent. And, well, "has flying" is just as bad as "has indestructible" and has been around forever.

Oh; doh - it also goes incredibly infinite with ANY cost-free self-sac creature and its own first +1 ability. How did I fail to notice that?

That feels a bit less fair; but, well... it is four mana and five turns to do that.

Her ultimate is vulnerable to things like Leyline of the Void and even Wheel of Sun and Moon, not to mention Grafdigger's Cage.