I believe that the Willow rework should focus more on her pyroMANIAC side, she is by far the most insane of all the characters and because of that her playstyle should revolve around staying insane instead of avoding it.
While sane she should have less bonus stats ( the fire immunity, resistance to overheat etc...) and fire, instead of rising her sanity should decrease it because being near fire woud make her obsession with it more intense.
While insane she would get a huge fire related buff like being fully inmune to it, being able to catch in fire and making her target catch on fire too etc.. this would also make Bernie have a bigger synergy with Willow protecting her from the shadows while she has fun with her fire and raising her sanity when the player is done.
Of course something has to be done with the insanity noises that many players can`t stand (myself included), she could get her own insanity noises and screen filter (they could be fire based like ashes dissipating or flames flickering I dunno).
I think something that could increase the value of Willow for her team could be making some weaker explosives that wouldn´t catch anything on fire but could serve for area damage, maybe a version of the flask from the Forge and some firecrackers that could serve as traps to stun lock enemys and that will not affect Willow as long as she is insane.
TL DR: Willow should get benefits for being insane and some cool explosives to share with her team.