2D for older franchises, similar to Sonic Mania. Just keep the spirit of the original Genesis games intact while giving new levels and features and such. Streets of Rage, Golden Axe, Comix Zone, Vector Man, you get the idea. For 3D, Saturn or Dreamcast franchises. It's really that simple. Go with what works. Don't reinvent it. Enhance it, but don't say oh that was ok for the time but we've gotta appeal to today's hold you hand demographic. :p People actually complained Sonic Mania had challenge. Get good, guys. XD