For starters, don't try to reinvent the wheel. Sometimes less is more. Think it simple. For example, pick a basic gameplay style you like (a CvS groove for example) and go from there, change a thing or two and see what works and what doesn't.The same goes for the roster. Start with a base roster of 10 characters, and if everything goes as planned you can add more down the line. If I were to suggest I'd go with three characters from Street Fighter and two from Darkstalkers, leaving five characters from any other franchises you'd want to add.