Throw the hybrid lovers a bone, add 1D6 of the appropriate type per core to melee and ranged damage.
It's nothing overpowered and it still won't make a caster with some melee class levels that good, but it will be flavorful and it gives them at least some reward for their investment.
Some people just think it's cool to swing a sword and cast fireball SLA's, why not make it less bad? 6D6 is not breaking any melee builds (paladin gets 7D6 light and it's not exactly the reason to be in that tree)
And 21 damage is not going to make casters suddenly swing a lot (just gives them something when they are OOM)
And if some melee wih, say, a 14/5 split with a level to spare chooses sorcerer for 1D6 typed damage for 1AP, good for them :-)
You don't even need to make it a toggle; they are on their way to become an elemental so if they don't want to heal the fire thingy, they shouldn't touch it.
I'd say scale it with MP/RP. You could make it scale with spell power (and start with 1 instead of 1D6) but that would make it more narrow.