I actually wasn't upset on Lady Tremaine, The Evil Queen and Frollo not being bosses because they lacked the combat, but I was upset on not fighting CLU.
Tbh, I don't think Bellwether would make for a good boss in the next game because all she did was relied on her Ram Guards doing the work and temporarily use a Night Howler Pistol (which turns out was actually switched out with Blueberries instead of the serum). The only thing that would allow her to be a boss is she either has a heartless assisting her like what Clayton and Barbossa had it or she succumbs to darkness and turns into a heartless herself like what happened with Gothel and Hans.
Or they can have the Zootopia world set after the events of the movie.