Right at the start when the countdown is underway, look down the track at the start line, you see where the checkered banner is going across the track? Look at whats holding it up, It's two of those eye things, the boss from the original game. You can see it much better in game if you just drive upto it and look. But its definately there, when I played with Dread and Shadz I spotted it and they were both saying "Where?" so I said, go to the start grid and spot where I am, then look at the things holding up the banner. They both spotted it.
The Doomsday boss however, it's all over the place, one flies over the track just after the first bend, then theres a couple more later on, you see another when you do the jump/go past Ristar thats flying around in space.
Now I see it, and there I was constantly looking for that giant robot from Eggman that was the boss from Act 2.
As for Genesis characters, at least Ristar made it in as a cameo.
I was already glad we got Alex Kidd, Opa-Opa and the Bonanza Brothers, even though those first two were more from the Master System era.