RF modules (LoRa et al)

Yes, that kind of interface looks good - it might even allow something like the MQTT module work on top of it if people wanted.

For decent battery life you'd definitely want at least the decoder to sit inside the radio itself, but there is the option of sticking all the radio implementation on a second micro. I've seen a few devices based on the CC1101 (SRF and eRIC) - but that seems like it might still end up being quite expensive.