The spell came out without a lvl25 tier and i think that there should really be one added, it seems unfinished to me. I think it should be fairly powerful also for having to be in a primal destiny to get the druid caster level that high, something fairly epic and very fiery.
+6 Scimitar 2.5 base damage, greater incineration, blazing, blinding embers, fiery detonation. possibly a magma gaurd effect but for a weapon....i dont believe that exists yet.
Another idea would be a 1.5 or times wisdom modifier for attack and damage, it could be interesting but possibly op.
A plethora of other ideas lie in the base dice and crit threat/multi.
I think the hardest part about deciding what it should be is that swailing blade is already so similar.