I did not yet started coding, so I hope it will be still possible with good old ATmega328P. Also not sure if this application has need for RTOS, or if it is doable only in as standard Arduino sketch.
Anyway it has possibility to expand the zones by 8, and the connection can be established via wire(RS485) or radio(RFM69). It has onboard relay and similar power detection circuit as gateway. There are only 2 free pins left that are taken out to allow some user application like additional relay board.
Gateway firmware is already able to allow 2 such boards to be connected.