I don't think you've yet covered each 2D Sonic game yet, G.E.R., and, should more show up, well, they'll be waiting.
Sure, Sonic Rivals was a rather tough challenge, but I had good fun winning each stage by being first place ahead of my rival opponents. The one irritating annoyance was the rivals' rubber banding in its programming that made whoever was my rival catch up quite quick, no matter how fast I got.
I get your perspective, TerraEsperZ, but I still feel it'll be worth the effort in mapping them. It won't be no trouble for mappers like any of you to accomplish.
And, I've not seen any word of their length comparable to prior 2D Sonic games, or Sonic inspired games, such as Polyroll, Spark the Electric Jester, or even the Sonic 3-formulated indie game, Freedom Planet, whose maps are all still yet to be seen, so to speak.
They've gotta be done sooner or later, and people can't wait for it too long. I'm sure any of you can do it, if you're up for it that is, for I have confidence in you.