As a Willow main I can concur that her current benefits are really lukewarm and not very interesting, and her Rebalance benefits are kind of exactly the same?
Looking at Willow's Rebalance it seems like the developers kind of forgot what the point of Willow is, which is fire. The idea of having an army of Bernies fighting enemies is cool, but also ridiculous and not consistent with Willow's character; which is fire. All the fire benefits they want to give her are really forgettable so im'ma just put my idea for a rebalance here since it's what I was hoping they'd do. The main idea here is fire.
Benefits :
Willow no longer takes damage from fire, and heals slightly when she is on fire.
Enemies that die to fires caused by Willow drop their normal items instead of ashes.
Willow gains sanity from starting a fire, standing near fire, and the insanity aura of enemies doesn't effect Willow if they are currently on fire.
Fire heats Willow up faster if she is cold, and she takes less damage from overheating.
Willow doesn't lose sanity when using the Fire Staff.
Downsides :
Putting a fire out reduces Willow's sanity, Willow gradually loses sanity while it is raining or snowing.
Willow freezes easier, takes double damage from freezing and loses sanity while freezing.
Willow loses one sanity each time she eats something that hasn't been exposed to fire, as in cooked.
Willow has a Fire meter which passively decreases, and is increased each time she starts a fire. The size of the object burned determines how much starting the fire replenishes the meter, and standing near fires slightly increases it. Willow gradually begins freezing, and losing sanity, if the meter gets too low; and this freezing comes on faster the lower the meter gets from that point.
There's probably more that could be added to this but that's about all I got. Basically the idea is that Willow likes fire and doesn't like the cold. Her main benefit is that she can weaponize fire in a way other characters can't, and she has various modifiers which make her gain a lot of sanity and lose a lot of sanity. Because she's obsessed with fire, she has a fire meter which makes her have to keep making fires to stop from going into her form of withdrawal.
Fire.