A wireless sensor node, powered by a rechargeable battery and a solar cell. For outdoor agricultural use.
The only interesting item here (and the one that implies the use of an RTC) is the mesh networking. I'm considering the idea of the devices only being on for the first few milliseconds of each second, or 10 seconds, or minute etc. It depends largely on the accuracy of RTC sync systems and the amount of time needed to establish communications in a network in order to any perform any required housekeeping. List of considerations:
It may be easiest just to adapt something like RF24Mesh
Now the hard bit is over. All that's left is the solar integration.
The SX1278 seems like an interesting chip worth investigating.