Problem with this would be that the Joycons would become even smaller, and they are too small already for people without small hands. They could be fixed to that Switch mini - but wouldn't that defeat the Switching mechanic?
Hence why I think they will bring one handheld with fixed Joycons- which can be switched into a Smartphone. A smaller Switch would be in that territory anyway sizewise, so why not try and fuse the best of both worlds together? Nintendo could change a portion of the eShop into an appstore (because no android, hence no app store) to bring productivity apps as well as some smartphone games and make another Smartphone unnecessary if you have that kind of Switch phone = Multimillion seller guaranteed.
I was also thinking about fixed controls, if you want Switching mehanic than go buy regular Switch, point of Mini/Pocket version is to have smaller or whats more important lower price point Switch just for handheld play.
I dont think Nintendo would release smartphone, even Switch is totally gaming device currently dont offer any other service.