I expect a Switch Mini to happen, but not until the next Switch iteration.
I think that in something like two years, we're bound to see a Switch TV: a dirt-cheap, tiny little box that you can buy to play the Switch's library at home, once the Switch has a stronger library and once Nintendo's more thoroughly sold people on the hybrid angle.
Sometime after that, I'm expecting a new Switch iteration that's the same size as the current version, with a more efficient GPU (like X1's successor) that renders 'docked' visuals on an inbuilt 1080p screen.
At that point I think a Switch Mini will be in the cards. With a more efficient GPU, it's possible that Nintendo could shrink the size of the entire device, battery included, and still render games w/'undocked' performance on a smaller 720p screen, with acceptable battery life. And it could function exactly as any other Switch when docked.