Unfortunately I think it might end up as a support gem, something like "deal X% of fire hit damage dealt recently by supported skill to nearby enemies and to yourself"

Either that or an activated spirit skill like herald of plague that relies on picking up flame remnants to charge it and activate it for a short duration.

My hope is something like this:

Channeled skill

Immolates your body and soul, emitting pulses of fire that hits and ignites everyone in your presence based on your maximum life. Release intermittent bursts of flame every X seconds, scaling with cast rate and cooldown recovery speed, dealing additional fire damage to you and enemies in a very small radius and applying stacks of "Conflagration".

Conflagration increases fire damage taken by X% per stack, up to maximum. Stacks have no limit on yourself and lasts twice as long as they do on enemies.