Oversimplified description:
I have a pfsense device whose sole function is to do NAT between a 10.x.x.x/24 network, and a 192.168.x.x/24 network.  I connected the WAN side to 10.x.x.x/24, and the LAN is 192.168.x.x/24.  Ideally, I would like a range of IP's to be 1:1 nat'd in both directions, and all traffic passed.  But since that was proving more difficult than expected, I'm aiming low now, just trying to get SOMEthing to work.

I went into NAT.  No port forwarding.  No 1:1.  Outbound automatic.
I went into Interfaces / WAN.  Clear the checkboxes for blocking private and bogon networks.
I went into Firewall Rules.  The WAN and LAN interfaces each have one rule:  permit everything.

By using packet capture on pfsense I monitor traffic while I do this:
Using a 192.168.x.x client, I set static route to the 10.x.x.x network via the pfsense box.  I ping a 10.x.x.x client.
In the packet capture, I see the ICMP Echo Request come in the LAN interface.  I see the Echo Request go out the WAN interface, successfully NAT'd to the 10.x.x.x IP address of pfsense.  I see the Echo Reply come back.  But the Echo Reply does not go to the 192.168.x.x network.  It seems pfsense NAT is forgetting about the connection, or firewall blocking.

I go to System Logs / Firewall.  (Clear the log, repeat the above tests).  Nothing new appearing in the log.