The Star Fox Grand Prix is something I've been really curious about. Strong rumors appear out of nowhere, and it gets brought back up every now and then. I'm not too sure what to think about it.
On one hand, I hope it's real. I'd love to see a new Star Fox title on Switch, and although I wasn't a fan of the idea at first, I read more into it and it could work quite well. The way it sounds is that it's similar to Diddy Kong racing. Imagine going through Corneria in Star Fox 64, only Falco, Peppy, and Slippy are controlled by other players. You're also making it to the boss together, but instead of it being a cooperative experience, it's a competitive one where you can even shoot at your 'teammates'. I think a game like that on a mobile console like the Switch can actually do quite well.
On the other hand, I don't think that this is the best route for the series to continue down. If it's a spin-off game that isn't really meant to define the future of the series, than I'm more okay with it. I think Star Fox needs to go down the Star Fox: Assault route. Bring in a solid plot that introduces a new enemy, continues to build off an existing timeline, and brings in a mix of vehicle and on-foot combat. Or they could go the Starlink route and go open-world, allowing you to travel the Lylat System, like that one rumored game, actually called Lylat System if I'm not mistaken.
Either way, if it's real, I really, really, hope it sells well. At this point, I won't complain about where the series goes, as long as it's successful. Gets the series off its feet, and one step closer to becoming a household name. Just like what Miyamoto wants it to be.