Gimmick idea: Cross-game bonus

[Gorilli09]

What is cross-game bonus?
Cross-game bonus is where you earn a bonus in one game if you have save data for another game.

How would this work?
If you have Sonic Robo Blast 2 and Sonic Robo Blast 2 Kart on your computer, you'll unlock Amy and Fang if you unlocked them in Sonic Robo Blast 2, and new SRB2-themed tracks in Sonic Robo Blast 2 Kart. I know that Amy and Fang aren't officially in Sonic Robo Blast 2 Kart, but this is just an idea.

Edit: If you have Dr. Robotnik's Ring Racers and Sonic Robo Blast 2 Kart, you'll unlock Sonic Robo Blast 2 Kart tracks early in Dr. Robotnik's Ring Racers.

More info

Old Save Bonus - TV Tropes

The ability to use old save data (or in rare cases, the presence of the game itself) from one game in a different game (often a direct sequel, though unrelated examples aren't unheard of) for a bonus. Mostly, this means a tiny change (such as an …

tvtropes.org

 

[Dragonick]

I think a feature like this would be pretty cool, although probably better fitting Ring Racers, since Kart's confirmed to no longer be in active development, and since Kart isn't exactly noted for singleplayer content.
For a sequel to a game with its title in the name, Ring Racers doesn't have a whole lot referencing vanilla SRB2, so it'd be neat to see some unlockables in that vein. I promise those rhymes weren't intentional.
There is the issue that not everyone playing Ring Racers would necessarily want to play a completely different game to complete it fully, so perhaps they could be made secondary unlock requirements, as in a bonus for those most likely to get the references.