Creating New Campaign Map in Rome Remastered
The underlying map structure in Total War: Rome Remastered is exactly the same as for the original game and can still be changed via text file and tga image alterations, however, the visual element of the campaign map is now provided by a separate 3D mesh and textures.
Mapping Components
Any base map that loads under RTW should load under RR including using the same map.rwm. For general advice on making a Rome Total War map see the Mapping Index.
Mesh Components
Mesh Heightmap bins
Using a data\terrain\campaign\descr_map_tiles.txt file with differently named .cas and .bin files requested than in the original game will trigger the game to write new .bin files, if these files don't already exist in the mod-folder they will be written in:
DATA\terrain\campaign\heightmap
That's the main game data folder not the mod-folder, cut and paste them from there to your mod-folder! Cut, because if they overwrote your original game files, you will need to let the vanilla game recreate them based on the correct map.
Ideally, name your new piece .cas files differently from the vanilla ones to avoid any accidental overwriting. Currently the game seems to require 104x files listed in descr_map_tiles.txt, however, these can just be small duplicated files hidden under the frame if you don't actually need them.