While this mod adds a lot I currently feel the lack of a generic version of the republic with them only being formable in historic locations takes a bit away from the potential fun.

I have a suggestion as to how you could handle it in a sensible way, with a Game Rule for Fantasy Republics. Something like:
Fantasy Republics
Historical Only / Historical + AI / Fantastical / Fantastical + AI

The idea the AI part decides whether an AI also has the option to establish a Republic/ Merchant government, with the Historical one sticking to the original rules for establishment and the Fantastical allowing them to be established anywhere

That way the player could start a republic anywhere, but still allow players who only want a historical game to not have to see strange ahistorical ones.