I can think of a number of future improvements for the map generator. One is a four times bigger max mapsize, but that will probably need to wait until they have done some performance tuning.
Real forests (including option that they cover most of the map) and real mountains (including vertical cliff faces, side streams with waterfalls and whitewater (locations for watermill-powered industries).
Realistic distribution of mineral resources.
Import/export locations at the edge of the map. Can be a seaport on a lake at map edge or a customs warehouse on a pass at a map edge.
I wonder if a bigger map would really cause performance issues, if the number of towns and industries remained the same. We would have more space between towns, which will be a problem due to the 20 minutes limit, but I am struggling sometimes to fit some complex rail-junctions between cities, especially without X junctions possible. So I would welcome some more space between cities, would even be more realistic.
We don’t need bigger maps, we need different shapes. If they made maps that were rectangular instead of square, we could have some MUCH longer routes while keeping the same overall volume. This would be superb for anyone wanting to build a proper high speed line and maybe we’d actually have space to get to top speed!