leave a trail of flame in their path
A player on fire could randomly dribble droplets of fire on the floor (one every second or two). The droplets would spark to the side or fall to the floor and burn for a while wherever it hits. If that player gets hit, a few more droplets could spray from the impact location.
The droplets would not start other players on fire, so would only be for effect. If performance is an issue, the number of total droplets burning could be limited.