If I may suggest some ideas :
- there should be some maps with more little rocks as they slow moves but don't affect line of sight (ideal for low ranged weapons like javelins) ;
- there should be some maps with nothing on them, but with little elements to put on as we wish. (Does the words "customizable maps" are the correct ones for this idea ?) ;
- there should be some maps with walls (or some walls to add to the maps) ;
- there should be some maps with "holes" (I mean the opposite of hills) ;
- a new river set would be nice too, but with a river on a side and not crossing the middle of the field ;
- swamps or little water pools would be nice (some giants insects from the Goblins can't cross water... tactical effect guaranteed) ;
- bush / tall grass are cool, nearly as cool as little rocks ;
- roads are cool too (tactical effect guaranteed when moving a single hero along a road).
I think the best idea is a full customizable set (4 maps with nothing on them but lots of hills, holes, grass, swamps, waters, forests, rocks, roads...). Maybe a rule to add / remove elements (based on the number of * the highest general have and the scouting force or something).
I know, that's a lot of stuff to think about.
Sorry to give you headaches...