A wan is going to be any interface that can be used to get to other networks. You can nat or not nat to this wan connection. As mentioned already you have an asymmetrical problem putting this "wan" network of pfsense where there are devices..
If you want networks behind pfsense, and you want a "wan" network that will be used to get to networks not behind and directly attached to pfsense then this network should be a transit network..
Thats fine if all of these networks all connected physically on the same switch, you just need to make sure you break that switch up correctly at layer 2 to provide isolation.
Your going to run into asymmetrical problems as well if you just put all your networks behind pfsense on "lan" networks directly attached that use different gateway to get off their network other than pfsense. You would have to do host routing on every single host, etc.
Connect this pfsense to either your layer 3 or your edge with a transit network and correctly route.. Any network your going to put behind pfsense like this 192.168.100 should be isolated on their own layer 2 and use pfsense 192.168.100.x as their default gateway.