I have been looking more and more at multi maps; "all" that is needed is somewhere to store them and something to distinguish them apart. Originally we were talking about a a "Z" value to go with the plot X and Y values with a link in XML saying which Z referred to which terrain set probably by adding a new XML table and tag or tags in the terrain XML files to link the specific terrain/feature/bonus to specific maps.
Movement between maps would just be something like the current "Go To" button. By the way the "Go To" button needs fixing as it shows places you can't get to. For example land locked cities are showing up as destinations for ships
. This is another dll feature that I can't work on
. The one that I have seen, probably in Civ III, only showed places you can get to but showed the distance also.