I would love to see the maps become much, much more complex.
If for nothing else, for the aesthetics. Completely symmetrical maps are hard on the eyes after a while.
Strategy wise there are room for improvements as well. Height advantages, line of sight blockers, spell blocking terrain (not sure if the engine allows for this), move slower in water, and even destructible terrain.
If you want to get really fancy, some heroes could freeze rivers making heroes walk faster across them. There could be a portable bridge item (consumable) to allow people to walk across gaps, usable even in the current maps.
