RTF is a global modification that replaces all towns in the game.

actually, I've always been in favor of creating a faction of free cities, but they don't fit here. The only option is to combine Fortress and Naga, as Nival originally intended. This would free up one city.
This could be linked to the storyline in a very interesting way: I don't know how the Naga and Lizardmen will appear, but if we combine the factions, we can do it like this: the Necromancers were driven out of the Silver Cities, and the Cult of the Void sailed to the Misty Continent, where they drive out the Lizardmen and Gnolls. After that, the Lizardmen went to Hashima, where they will have to fight the pirate dwarves, Chimera mages, ice Naga, and free cities.