Sonic All Stars Racing Transformed was pretty fun to play considering it was a Wii U launch title. However, from my playing experiences and my bro, there's still technical issues that need to be balanced out for a sequel, such as the framerate w/ split screen, a bump up in resolution, better fine tuning of difficulty in the higher levels, and maybe a few changes to the track design to make them more fun than tedious.
By comparison, MK8 beats Sonic All Stars in almost every way, except for unlocking base content. That being said, I hope that they make a sequel to Sonic All Stars as it has a lot of potential.