[DrPraetorious]

There are several forge *sites*.

I don't think it is possible to build any national units in provinces without fortresses. AFAIK, this is pretty fundamental to how the game handles unit production. Likewise, it would require a major rewrite

Other than that, what you're suggesting is obviously out of the scope of current modability - but I don't think it would be a problem in principle. You'd need to introduce some new commands -

-- Building modding
#selectbuilding <nbr>

This enables you to select buildings other than 1 (the temple) and 2 (the lab).

#defaultcost <cost>
#defaultpic <file>
#end
Would be all you'd need to set requirements for your new building. Other than that, you need site commands to give the building stuff it can do - like add recruitable units and so forth. Set the cost to a negative number to make the building disabled by default.

-- In nation modding, add:
#buildingcost <nbr> <cost>

To change the cost of a given building for a given nation. If 0 is temple and 1 is lab, that'd enable us to mod those easily.

-- And in unit modding, add:
#customreq <nbr>

Where <nbr> is the building number.

I'm sure you can see how you'd use this to add Forges, and units that require forges (which would then have an increased resource cost.) They'd require a Fortification, as well, unless everyone who build a Forge got them.