I think more than anything, the character roster. It has been kinda a problem since Mario Kart Wii, and it only aggravated with every installment, culminating in... Pink Gold Peach in MK8 lmao
MK8DX iiiiiiiiis putting some improvement on the matter, but it's basically only fixing the situation to be tolerable, by only adding veteran characters (and yet still no Birdo or Diddy). OK, the guest characters are really cool, but that's what they are... Guests.
I mean, 3D World's representation in MK8 really shouldn't be limited to Cat Peach. I don't have anything against her inclusion (in fact, I actually like her and Tanooki Mario), but when even goddang Mario Tennis Ultra Smash did better by adding Sprixie, there's a huge problem. I'm still waiting to race as Plessie or Motley Bossblob... Heck if they wanted to put more reskin characters, why not Meowser?
It's weird that this is happening now, when you think about it: Double Dash covered Sunshine by adding King Boo and Petey; Wii covered Galaxy by adding Rosalina; and 8... Represented 3D World by adding Cat Peach... As DLC...
Another nice way they could beef up the roster is by making nice references to the Mario series's past. As in, adding the likes of Wart or Foreman Spike to the roster. It'd be unexpected and would diversify the selection a lot.
And don't even get me started how E. Gadd has never been playable, even after a lot of Luigi's Mansion-inspired courses and Dark Moon.