A piconet is essentially an afterthought to assemble a ragbag of stuff that already has bluetooth in it. I submit that you would be better off properly planning and designing a system from the outset. An NRF24 network has about the same price and performance as bluetooth but is designed with this sort of thing in mind. An Arduino base-station could handle the NRFs, talk to the outside world, handle the WiFI weather station, and even talk to the odd bluetooth device as well.