Hi,
Designing a circuit means you need specifications first:
* hat is the max load current
* what's the desired max voltage drop at the Mosfet
Btw: I like the idea, that GND is the common reference point for all connected systems.
But if you switch the GND, other systems can't refer to it, therefore I recommend to switch high side power supply line.
The easiest way is to use a P-ch Mosfet.
If you need some protection (overcurrent...): there are ready to buy power supply switches.
Klaus