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.