In addition to the rest of the ideas I have for communication I want to add one more thing; a flexible software serial header.

This is to serve as an extra port for communication and control of a slave unit's Nano. This port will mirror everything sent monitored in Nano's code. this can be used in severbal ways, such as:

USB serial cable - for streaming data to a serial window on a PC/Laptop serial monitor, and for manually updating thresholds.

With a Bluetooth module - to allow the same as above but perhaps with a phone/tablet app

With an ESP8266 module - an ESP module could be used in many ways; to host a basic standalone webserver in the bounce of a master unit, to allow wireless comms as the methods above, or even to stream data to IoT sites such as thingspeak :-)