Mission mode like DKR would be fantastic. That (and vehicle options besides just karts) was the thing that really separated DKR from MK64 back in the day. I think it's high time after so many years that Mario Kart finally steps up and gets a huge upgrade to the series and adds a missions mode.
If they do 24 kart races, mission mode, and integrate a much deeper level of other Nintendo IP into MK9 that would really be a big step up from everything that has come before. Especially with the next Smash likely many years away, Mario Kart is gonna be THE defining multiplayer game on Switch 2 for most of the gen and it's obviously gonna be one of the major enticements for gamers to buy the system in the first year so I really hope Nintendo is pulling out all the stops so that it's not just another iteration of a really fun kart game but something truly impressive and new for the series.