Review this brand idea:
Make his ult bounce if and only if it hits a target that is already aflame before it lands, with no bounce limit.
This mandates an aoe combo beforehand in order to "kindle" his ult. Slow travel time allows him to reactively "stoke" the flame if it bounces towards non aflame target.
Because the ult itself sets targets aflame, If he is careful to keep the fire stoked and takes care of it the flame will begin to burn on its own, just like a real fire.