There was a few things they screwed up on:
1. Driving feel. Just doesn't feel as polished as Mario Kart games. Not that it should feel the same, but you can tell when something feels more polished for sure.
2. Fix the water controls, I dunno how many times I screwed up a race cuz I landed on a wave and it just totally screwed me up. It shouldn't be 'chance' that you will get screwed up when entering the water or water driving.
3. Add something to make flights more entertaining. They just felt a bit plane.
4. Animation needs to improve.
5. Don't force each franchise to have the same look (like people said, Jet Set Radio) just because you want everything to be 'the same', if you wanted everything to be 'the same artsyle' you should have stuck with Sonic only. Embrace the uniqueness of each SEGA franchise. Thats like celebrating everyones culture and but everyone has to be white (obviously not as extreme as that).
6. DLC thats fairly price and distributed more fairly. See: PC has most free DLC. SEGA should team up with each manufacture and make DLC exclusive. For example: PC + Steam = free HL character. Nintendo = Free Mario or something DLC, PS4 = free Jak and Daxter, Xbone = Free Battletoads (or Banjo ;0 ) DLC.
7. Come up with multiplayer mini-games outside of racing (See balloon battle in OG Mario Kart).
8. Power-ups should be more unique to SEGA's history.
I can go ahead and add more franchises should be included (even if its cameos in stages) but as SEGA fans we will never have enough SEGA references ever...