What I think represents the "next" for the series
- DLC Expansions a few months after the launch: basically, the MK8's model used again. It's working fantastically, why couldn't they do it again. Also, we must take note of the "NintendOS" plans by Nintendo: if both handhelds and consoles will really support the same games, we won't need a Mario Kart game each 2/3 years (Mario Kart: Super Circuit - 2001; Mario Kart: Double Dash - 2003; Mario Kart DS - 2005; Mario Kart Wii - 2008; Mario Kart 7 - 2011; Mario Kart 8 - 2014) in order to support as soon as possible a brand new console (wheter it's a portable or a home console), it'll probably be more like 4-5 years between two full new iterations of Mario Kart, while the Mario Kart team will have the possibility on working on other, different projects, alongisde the DLCs. And what's the best way you can give a long life to a product like Mario Kart, that can be easily expanded for a few years without getting a brand new iteration? With expansions, of course!
- Non-Mario characters / tracks being a bigger presence in the game: as said by others, probably they will be more like the "big attractions" of the expansions, and not in the main game already. Even if, IMHO, it could be possible that one/max two non-Mario characters / tracks are in the game right from launch...we'll have to see.
- Better online features. Evolving on the bases from Mario Kart 8 and adding more "modern" online play features should be done. This includes better sharing functions.
- A separate single player campaign from the usual Grand Prix. I didn't play Diddy Kong Racing when I was little (no N64 at the time), but I played another (AMAZING) kart game that had a very good single campaign, that was a Story Mode, complete with storyline, an overworld to travel in, bosses, etc.etc. Of course, I'm talking about the majestic Crash Team Racing. I want something similar to that (and DKR, since it has a similar concept for the Story Mode IIRC) in Mario Kart. Maybe, in addition to that, something similar to the Mission Mode from MKDS that's already been mentioned here several times (I don't remember about it that much, since I didn't play MK DS that much, sorry ;_; ).
- 200cc from the start. I don't think anything else should be added.
- The return of a legit battle mode. MK8 really lacks in this.
- What about the gameplay mechanics themselves? Eh, MK7 introduced underwater driving and gliding, while MK8 introduced 0 gravity...eeh, that's kind of difficult...maybe the actual possibility of flying in specific sections of stages (like Sonic All-Star Racing: Trasformed)? Combinable items? First two that I thought about. Don't know what else would be considered a good addition gameplay-wise, right now.