Bad grip vehicles (C, D, occasional B like all 8 of my vehicles in GX) are amazing if you hold L and R the whole time... They control smoothly and they are amazingly well built/fast as well.
I want a few things for it. First, the amazing graphics of GX have to be made even better. Second, bring back some of the themes (Big Blue, Shotgun Kiss, Planet Colors, Mute City [which everyone hates, but I LOVED it] and more. Third, a completely revamped vehicle editor. It wasn't bad, but I want my vehicles to look just as cool as the stock ones, not a horrible mess of a car. Fourth, a well built track editor, where you can choose what theme you want your race to be (Aeropolis [or something like that...], Mute City, etc.). And last, an online Multiplayer that supports at least 4 people. I would enjoy it at around 10-15, but 30 is asking for too much.
GX was an amazing game that I'll love forever. I hope F-Zero "WX" follows the same route.