I was primarily asking about this from a hypothetical/design perspective, but if I were hoping to play such a character, I think I could see a refluffed flaming sphere as a reasonable compromise. An instantaneous spell, like burning hands, wouldn't really have either the flavor or the mechanics.
Consider this...
DM: The goblins race in, howling for your blood.
Pl3 (Bob the Rogue): I dive into the shadows. <rolls Stealth> 18.
DM: nods. The goblins don't notice you hide. Two of the goblins are heading for John the Fighter. Two more branch off to rush Louis the Mage.
Pl1 (John the Fighter): I attack with my longsword <roll> Yes! Adjusted 17! <rolls damage> 7hp.
DM: The goblin drops. Louis, you're up before the goblins attack.
Pl2 (Louis the "Summoner/Elementalist/Whatever" Mage): "I call the Flaming Serpent of Sephirot to scorch the evil of my enemies!" <rolls> 12 damage!
DM: The conjuror casts his diagram and a writhing serpent of fire washes out before you, easily striking the two goblins coming for you...annnnd...<checks spell> 15 cone, huh? Yeah, you can catch the third goblin about to attack John. <rolls saves> With shrieks of agony and fear as the writhing serpent streaks through one goblin after the other, leaving charred corpses in its wake. After the third goblin falls, the snake rise into the air, coils itself into a tight circle before the flames flare and disperse, leaving no trace of their existence.
Pl3: Woah! Louis!?
Pl1: That was some fancy spellwork, mage.
Pl3: For real! What the hell was THAT?!
DM: He cast Burning Hands.