I like the card, if anything it adds more variety in creature choice. The hard part is it surviving that 1 turn, but then you should have no problem with it. Sanctuary_Firestal_this ftw
Variety in creature choice? This is, at best, a more expensive Phoenix (at worse, a less expensive Dragon).
Also, I'd never replace a Fire Tower, a Sanctuary, a Miracle, a Reflective Shield, a Fire Lance, a Rage Elixer, a Fire Storm, a Fire Shield, or a Deflagration for a Seraph. Seraph is just too fragile the turn it's out to reliably aid in a win-condition, while the cards currently in Firestall are far more reliable.
Last but not least, it's hard to truly synergize with a card that only affects itself, and I'd rather have a card that creates new combos take the role of "a new [Fire] card."