So an adjustment to my earlier suggestion that Hyoton combo off Fuma.... The simplest solution to my earlier suggestion is just to increase the potency of the move, and maybe remove the bind (but leave it as is for PvP content)

If you increase the potency to 260, then you will have a single target Ten-Chi-Jin combo that is mildly different from an AOE version as Hyoton will now have a higher potency on a single target by 10 potency than Katon, and you'll still be able to Katon for an AOE Ten-Chi-Jin. That kind of buff will still leave the skill 100 potency less than Raiton, so it won't change anything outside of the combo, but will now give the skill a place in your rotation.