As far as I know in order to play PokéForce you will need roms of FRLG, HGSS, and BW/BW2
Will these games be needed only for assets, and monsters, or do you want to set the entire/majority of the game into Johto/Kanto/Unova?
I thought that you could also consider to make most of the game assets (such as overworld tilesets for example) simply by yourselves, so that IN CASE you want to set the game entirely in a fanmade world/region, you would just need BW in order to add monsters (or XY/SM in case you want to add fairy type as well), and nothing else
Will these games be needed only for assets, and monsters, or do you want to set the entire/majority of the game into Johto/Kanto/Unova?
I thought that you could also consider to make most of the game assets (such as overworld tilesets for example) simply by yourselves, so that IN CASE you want to set the game entirely in a fanmade world/region, you would just need BW in order to add monsters (or XY/SM in case you want to add fairy type as well), and nothing else