Burst of Flame is inconsistent because it relies on environmental fire sources (and because there are never torches on the south wall of a room, given the 3d texture of the map).
But when you had the flaming tempest epic talent (I don't remember what it was actually called) this allowed producing your own enduring flaming effects, and then bursting off of them.
I'd love to see this kind of mechanic return, instead of the "summon ranged fire ally". Maybe if flaming sphere made some flaming clouds, perhaps a bolt line of them or range targeted clouds (smite targeted?), instead of shooting projectiles this would bring the two ideas together.
Or, tier three is Summon Flame Vortex, with level two increasing the number of fire melee allies summoned (from 1 to 2? 2 to 3?), and you can Burst of Flame off of them. Probably, they would be need to be used up by this, to prevent *too much* synergy.