I think second chance is going to be a huge card in arena, but personally i don't see the most use for holy mages. Imo this is going to be the new thoughtspore enchantment. Think about it, you can summon a thoughtspore early, knowing its likely going to die and then when it does you get to resummon it on the spot AND change the spell! Sounds like awin win for me.
There certainly is potential but
An issue I see here is that it will cost the FM 4 SBP's to (possibly) use this trick, he can add two Thoughtspores for that amount of sbp's.
Another potential problem : the FM usually has mana to spare later in the game but early on you may not have the 8 mana available to revive him and keeping 8 mana to the side early on may hurt your build-up.
Another card though that's been wanting to get into my FM book to help the TS ( but hasn't succeeded yet ) is
Divine Reversal. At 2 sbp's this is much more affordable and in a way more effective than a Block ( Unavoidable weakness ) or Brace Yourself ( critical damage weakness ).
Since it prevents the first two damage you must already deal 9 damage to the TS to take it out with 1 hit.
Since it heals another two after the attack this card can effectively neutralize 4 damage, normal and/or critical damage doesn't matter nor unavoidable or not.
As an example, let's say an opponent makes a super roll of 7 critical damage with an unavoidable attack :
Block fails and your TS is dead
Brace yourself fails with the same dead TS result.
Divine Reversal prevents the first 2 damage, then takes 5 damage and heals another 2 of them for a net result of 3 damage and 4 life left. Not bad hey after such a magic roll