i like it a lot, but like someone said, don't make it the Annie clone.
1) Obviously Tweak the numbers, they are quite OP with the 100% reduction in magic resist....
2) His Q skill i guess or Burning Death should have a ramp up effect. Either have it apply as a self-buff which after a certain point yields greater rewards. (stacking Dot effect, i'm thinking like Ignite from WoW) Can crit? Or ignores magic resist or something. ( i kind of like that theme he has going on)
3) his passive is cool, but kind of plain. I'd rather have it fit more along the magic resistance reduction theme u have going on here. Idea I thought was, "Heat of Battle: Spells against you grant you X magic resistance, while your damage spells reduce X" OR (Have it based on ARMOR. This would add another dynamic to the hero. He can be used to counter high armor/resist chars, even if they dont go solely magic resist)
4) His ult is like gangplanks. Have his ult be a passive, which makes his spells deal AoE damage or even better check this out: "Unquenchable Flames: Bubble's spells will jump to a nearby enemy champion dealing 30/50/70% of the damage dealt". This gives him that AoE usefulness in team fights, while still allowing him to be more of a single target burner, pun not intended.