It seems to me that you would just use the WAN interface for one of
the subnets, and turn off NAT.

It is a possibility, but I don't want to do it. For example: wan
interface needs gateway ip. Why I want a gateway IP between two
subnets? Perhaps each interface could be assigned the type as for the
wan interface.

I don't believe you can rename the interfaces.  

It would be a nice feature. I think. 

No, there is no SSH or console, the closest thing to it would be
exec.php which allows you to run shell commands.

Fine, beauty option. 

I think  you're able to use up to four interfaces (LAN, WAN, OPT1, OPT2).  

UP to four is the limit? 

If you're only routing between the two subnets, you might get decent
throughput.  With NAT on, I get about 30mbit of throughput from a 667
MHz Celeron, but I haven't tried testing with NAT off.

Sure, I only want route and firewall the two or more subnets. I don't
want NAT. I only want an internal router / firewall. Is monowall right
for me?

Thanks.