There are "lots of top-class scoring programs that import MusicXML," but do many of your main group of potential customers use "top-class" programs?
I can't give you an answer on that, but I suspect that a .mid to abc, or a "front end" plugin for some <$60 program, even if people needed to get the program, might find a larger market base accessible to you.
Even a "dirty .mid to clean .mid" utility might make more sense. Nearly all the people around here do have some program that imports .mid to notation.
A "simple(?)" utility, sort of like MidiText/Text2Mid that you could drag a .mid onto and get back a "cleaner/quantized" .mid would seem to me more likely to get used by larger numbers of the people here. Since a user would likely want to play with settings, maybe a program that actually opens the file and lets you look at the result would be needed. Any such program would likely need to display the notation, but the simpler the better. If you can find a preset degree of quantization that seems to work well, the drag-and-drop converter would be great.
For the follow-on, the utility to "put a little swing" into a ".mid from score" should probably be a separate program.
I do suspect that most of your potential market is in people who don't have one of the "good" (read $$$$$) programs. Since most of those programs do accept "plugins," you might want to look at the market possibilities for something for one of them; but that's a whole different culture to deal with.