Been meaning to jump back in an RP on this site, and considering I pretty much inject Undertale into my bloodstream as part of my regular meals, I think it'd be a great place to start.

Personally, my vote goes to Overtale, and I'd be pretty much up for taking any role, though if it's GM, be warned that I don't consider myself very good at GMing. Any of the others you folks have suggested I'd be up for as well, though.