MK8 is essentially Nintendo's GTA V
Even if. So what?
A remake of Mario Kart GP which was never ported (officially) to home consoles, a new Double Dash, a Mario x Sonic Racing Transformed crossover. There's a lot of potential
They could easily bring all features from GP to MK8D. They can easily do crossover DLCs with franchises like Sonic. Pretty sure none of that will happen since, you know, Nintendo is Nintendo, but I
would prefer this stuff being included in the best and biggest MK to date rather than having a new series entry with a limited quantity of content.
Double Dash for what it's worth is a nice game I like coming back to. Then again I don't see a real point in it since we've got two item slots in MK8D.
No matter how you look at it, what I've written earlier is still my point: Any new MK entry could provide cool new mechanics but we would be stuck with a way smaller amount of drivers and especially tracks for a long time after release compared to MK8D. And if one thing bored me to death upon playing MK8 since the Wii U days up until the point the first DLC wave for MK8D dropped on the Switch, it's playing those same few tracks ever and ever again.
Smash Bros. Ultimate lacks from the same issue btw. A new entry could be refreshing but Ultimate will have the most fighters for many years to come in any Smash entry.
The only way I could think of how Nintendo could solve this issue with the next MK is providing either a track editor ("Mario Kart
Maker") and/or include randomly generated courses that an algorithm calculates newly before each cup. Then again that wouldn't match Nintendo's high quality standards of their current tracks obviously, depending on how good they make this feature.
tl;dr => Nintendo really has to come up with something amazing to have a product that screams "I'm better than MK8D with all its DLCs, buy me!" to us gamers, otherwise I'll gladly take more content for MK8D.