The Sonic Heroes mod is an awesome idea being done elsewhere. I'm wondering if a good place to start would be to do a similar "demake" with another title.
Maybe make Yakuza out of Shenmue lol
Or more realistically, perhaps re-skin a Racing game (such as Buggy Heat or Daytona USA, OR POD2 so an online version is doable) to replace the cars with famous Sega cars (the Hornet from Daytona USA, the Toyota Celica from Sega Rally, the Ferrari Testarossa from OutRun, Ferrari F355 from F355 Challenge, the F1 car from the early 90s Virtua Racing, Axel's taxi from Crazy Taxi, a truck from 18 Wheeler Pro Trucker, the Tomcat from AfterBurner, a buggy from Buggy Heat, the Twinkle Circuit buggy from Sonic Adventure, Eggmans Egg-o-Matic etc...)
You could also retexture the levels to give them aesthetics more akin to levels from Sega games.
I dunno how hard any of this is