Yeah, I get that you wanted to create a creature that is near-impossible to remove from the table once in play. But Quintessence is a much cheaper way of doing that. Eg. Phase Dragon already does what the Phoenix does except it doesn't drain your quanta (it cannot be killed/targetted).
Creatures are specifically designed to be difficult to remove once in play (can't remember who said that, or whether it was a general consensus from threads on the Ideas board). But it is a view I quite like.
At 5/7 for the card and 7/7 for the upgraded version, it would still take 1 to 4 Icebolts/Firebolts (depending on Quanta) or 1 Lighting Bolt and another creature control nuke. Players will understand that if they kill it, it will just comeback, albeit a lot easier to destroy in future. So in PvP players simply won't target it. Or they would Rewind it / use Eternity on it making you recast the original 10 or 12
![Fire :fire](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/fire18x18.png)
to put it back into play - as well as forcing you to redraw it/stifling your card advantage.
For me, Fire is a particularly interesting Element since it is based purely around offense and quick obliteration of your opponent. I particularly like the way two cards, Firebolt and Fahrenheit both do increasingly more damage, for the more
![Fire :fire](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/fire18x18.png)
Quanta you have in play. Immolation, so far, is the only card which boosts your Quanta by a lot - of course, sacrificing a creature in the process. Novas and Supernovas are the only similar alternative and they only give +1 / +2 to all Quanta respectively, making them valuable to Rainbow decks specifically.
Therefore I think if Phoenix (or other Fire creatures) were to have an on-death immolation effect, we would see Fire being a much more dangerous element (although, I'm not sure people want to see that, as it is already pretty dangerous!)