i think Caster is more effective then flamewave, because he is imune to fire and he can cast firewall and fireball near himself without being effected . The spells Help when your sorrounded or if you want to block a monsters movment or kill units that are protected by other units.
Would be nice to have an ability : 150% damage vs genies . Kinda miss those abilitys .
mark of fire could work too. after all inferno has the infernal fire perk.
lava breath could make sense too(not necessarily on 2 tiles). anti shooter unit.