I do realize the amount of devices and variations and the complexities, and yes all devices and all mode permutations cant be supported.
I am envisioning a single master only setup...
I was thinking along the lines of something along the following lines where one can
say a issue a write command to buzzer, LED matrix BOARD, or LCD board,Fans,
Or read and write commands to say externally independent heaters(chambers print bed extruders with their own closed loop monitors).
These commands to start and end of gcode and perhaps wouldnt need to add these changes to the arduino firmware.
M I2CAddress Data2Write : This will write the data to the address
M I2CAddress LoopReadTillTempReached :This will keep checking the Devices with where Temperature ha to be read the value and continue afterwords. The desire is to have something similar like the M109 command where it waits till the temperature is reached