Bokkun wouldn't be allowed. SEGA don't seem to want to use Shade and they're probably going for the most major characters first so it's a long way to go until Marine
In terms of Sonic Dash, Cream is the last character. That's it. Now, if they were to make a sequel, maybe they'd add even more (game) characters... oh, and more stages. I'm getting real sick of Seaside Hill.