So, earlier today, I noticed something: I play Mario Kart mode a LOT. Yet, I play Riders very little in comparison. Why is that, I thought? And I think I know why.
While Mario Kart is very accurate to the source material and is very easy to mod for (Possibly even easier than vanilla maps), Riders plays like a VERY watered down version of Riders. Tricks can't be done like they can in Riders 1, the courses lose their rotate-the-stick gimmicks (which were stupid anyway) and get them replaced with Zoom Tubes (which are worse because now you have NO control over yourself, when at least stick rotations made you go faster the more you spun), and the visuals are lacking in many of the levels. And while you decided to drop the Babylon levels for original ones, most of them are not that good (Ice Factory is downright awful).
Another thing is that in pretty much all of the courses, your type of character barely changes the way you play the course. Different stats is fine, but the shortcuts are meaningless in some of them as few of them make much of a difference in terms of how you play (especially in the custom stages where they're right next to eachother). For example, some of the maps in Riders have extra fly shortcuts because Fly types are the slowest, and obstacles were randomly placed in a lot of levels so Power types could easily crush them as the speed and fly types run into them, with fly types having a slight advantage with the easier turning. Here, however, the differing types don't add to the experience much at all.
In Mario Kart mode, the stats matter a little more, as the quicker acceleration and tighter turning of the fly karts, the top specs of the speed types, and the middle ground of the Power types add quite a bit of differentiation between types, and Sonic R we already know about (Though, those maps could have been tweaked to allow more fly/float/glide cuts since you can't thok anymore), leaving Riders with little difference for these types at all.
So in general, I think the "ported" riders maps can, for the most part, deal with at least some tweaks and liberties from the original designs. As for the Babylon courses, most of them need a lot of changes or even a few redos like Ice Factory. Riders would really benefit from this. It's pretty odd that Riders is probably the worst mode here, despite it being the NAMESAKE of the mod, and while it is obvious you poured lots of work and love into this mod, the level designs need some real work done to make it great. And while its great people are making MK maps, some guides on some of the mechanics for Riders mode and getting some custom maps for it would be great.