I've never been a fan of Mario Kart. I've always felt it's a series with uninspired tracks, awful controls, and eye-bleeding graphics. Yet somehow, Mario Kart Wii managed to overcome all of that and become my favourite Wii game. It's still got terrible graphics (I haven't seen square hands with no visible fingers since the Nintendo 64 days), but the new tracks are very creative and well-designed, the control is much more fluid and intuitive, and the vehicle and character selection is extensive and top-notch.
So, now that the series is this refined, where does it go from here? Well, Mario Kart Wii still has some issues. For one, the vehicles are terribly imbalanced. There are a select few that are viable in a race, all of which are bikes, that are simply too fast for everything else to catch them even with the items (on most tracks). Secondly, most of the retro tracks are downright terrible, especially with MKWii's much-improved game engine. So half the tracks in the game aren't even very fun. Thirdly, the battle mode was designed for online and has no options in the local portion. There are other points I could get into, such as needing a more robust online mode, but those points are either related to Wii hardware issues or not very relevant.
For these reasons, I don't think we need to see the series take a radically different direction just yet. The next console entry should have an abundance of new tracks (at least 24, but preferably 32), better vehicle, character, and item balance, and tons of gameplay options including things we've seen in past entries (like the double-karts). After we've had a truly definitive version of Mario Kart, then they can bring on the new direction.