Nintendo has market both Smash and MK 8 as “Ultimate” versions of their respective IPs in term of having lots of legacy content. Smash Ultimate has all fighters from previous games and almost all stages meanwhile MK has added with BCP a lot of retro tracks and various misses veteran racers like Diddy Kong, Birdo, etc.
You can of course adding more and more content like fighters/stages for Smash or racers/tracks to Mario Kart but I think maybe its getting a little of saturation.
So maybe they can try something more or less fresh. Some ideas..
- Mario Kart: Let it be a Nintendo Kart with half racers/tracks from Mario/DK/Yoshi IPs and the other with other Nintendo IPs like Zelda/AC/Splatoon/Pikmin/etc. Have a single player mode where you are in an open world map and must resolve challenges/missions, face races, bosses, etc. Aside from Grand Prix and Battle modes you have a new Battle Royale 64 player mode in same single player map.
- Smash: change gameplay to more 2.5D style, like Power Stone. Reduce roster to have more focus on Switch-era (new) titles (Example: FE fighter representation can be Marth, Byleth and Alear). Add a single player mode that is like Tekken Force, playing like a beat-em up with platform/exploration sections. Get back other single-player modes like Break the Targets.
Neither of these future ganes will have retro content like stages/tracks by start. Both of them will have DLC (paid) support for 3 years.
Considering Switch NG is probably getting full BC with Switch 1, I think Nintendo can find a way so both iterations can coexist with each other.
Do you want the classical “ultimate” experience? Buy Mario Kart 8 DX and Smash Ultimate
Do you want something new and fresh? Buy next Mario Kart and Smash titles.