A smartwatch with cellular modem is, at least from my point of view, currently not feasible to create based on free software. For such a device you are limited chipsets / SOCs which include all radios on the same silicon as the CPU and none of these comply to freedom requirements we want to see met.
What I could rather imagine is a small always on device like a watch which serves as a remote display with limited input and probably also as wireless security device (to unlock your primary device, store keys, authentication purposes etc.). But something like this is unlikely to be developed within the next year, so don’t hold your breath on it 
Cheers
nicole