I think it could be neat if (while aflame) there was a chance for nearby stuff in melee range (or maybe just the thing you are attacking) could take fire damage as well? Like for example if I set Ironside on fire, he could run around like a madman causing some collateral damage as well.
This could work as maybe a once/round thing for 2-4 damage (with reflex save to negate) within a radius or maybe even like a damage shield for 2 damage or so?
Like the disease AOE. Just take the code from that and apply it to combust with the effect rolling a d4 of fire damage as opposed to getting infected with something
(http://i985.photobucket.com/albums/ae335/angrykirby13/i_support_this_post.jpg)
Combust is already a direct damage second level spell with no save, and you want to make it stronger?!
It seems pretty minor, even including a saving throw to resist the spreading, and it would be so hilarious to watch in effect.
I don't even care about the damage, just seeing the animation get transferred constantly from one thing to another would be comedy gold!
Make it so it affected anyone irrespective of faction and it would be as dangerous to friend as foe. Which is very much like fire acts.
What if we cast combust on Ironside and while he was on fire, we've set Ironsides companion on fire and Ironsides companion and him cross paths? Double burn Ironside?
Might make a NAPALM oil equivalent item that acts this way. But not as a buff to this spell. Perhaps as a monster ability. Goblin Pyromancers / Orog sorcerors could use the buff to torture you guys further.
We have to be able to use it on ironside. Even if it's not attached to combust (The spell), being able to set ironside aflame and send him screaming at our enemies is an absolute must
Could make it a level 3 alternative, or a spell focus bonus too. Like if you have greater spell focus evocation. That would balance it out a bit, maybe.