I would love a Switch Fit. I liked the direction Nintendo was going with fitness products in the Wii era, but the Wii U failed to take it far enough. The vitality sensor and such seemed like an interesting direction. It’s a shame they couldn’t make it work. But to be fair, the Switch is a WAY more attractive console than the Wii U for fitness applications. The Joycons, if they can store fitness data, a wearable won’t be necessary - although maybe a combination of the Vitality sensor and a fitbit type deal.
I think the body fat content and such is something that was missing - not everyone is trying to lose weight, but almost everyone wants to raise muscle percentage and drop body fat. Also, I would LOVE if they incorporated Tabatas into the mix - as well as better support for non Wii-Fit exercise such as sprinting and weight resistance training; Wii Fit kind of had a general sort of “did this for X minutes” support.
Switch Fit can take it a lot further than basic weight loss - good for some people; but as far as gamers go, Nintendo fans are generally much lower on the obesity side than other fans due to being more active and true social (I imagine Xbox fans have the highest obesity since they’re the most “online social”).