Hm, does it have a map editor? I've been looking for something that can create BM-style maps for a long time now...
Way old -- and I know I'd love to get back to this (life hit me like a truck and proceeded to run me over repeatedly for the next few years), but in case you're curious for an answer...
It's an involved process of creating images and plain text files mapping features, elevations, water, etc to particular locations on the map files themselves.
http://forums.totalwar.org/vb/showthread.php?50437-Making-a-new-campaign-mapIf I recall correctly, it's basically the same process as this for Medieval II.
More resources:
http://www.twcenter.net/forums/tags.php?tag=creating+a+worldI'm trying to get into game creation myself, so I guess to that end this could still be put to the wayside. And I think I lost at least most of my work from the multiple moves since. Sorry for the necropost.
However, if there's still strong interest in something like this, maybe I'll hone some of my skills here with bits of free time now and then. This mod would actually focus on some of my weakest points I think; I am a software developer, and probably a rather pedestrian designer.
