Esker
Hero
I think it would be cool to make fighting styles more granular and let you pick up additional features as invocation-style abilities as you level up (and, like invocations, swap features out as you level). You could group the features into "paths" to guide players to selecting certain sets, but not necessarily require it; in other words, make them like Totem Barbarian features. This would let you make more fighting styles viable without introducing additional possibilities for multiclass cheese.