Keeping it within the realm of the possible, but still rarely thought of:
1. A way to address and alter gameplay interface elements. Allow the Upkeep indicator to be used as a resource box
Honestly, that's the biggest issue with the worldeditor right now for me. Allowing even a little flexibility in this department would also allow for tons of creative ideas to be realized. Tying the Upkeep indicator directly to the Food ratio is such a duct tape solution that I can't even begin to describe how frustrating it feels. If Blizzard would allow the Upkeep indicator to be used as an additional Resource box, with icon and all, that would definitely feel great.
Additionally, I'd be pretty happy to see the option to use different Resource icons for the different races.
2. A way to address and alter gameplay constants
Right now there is no way to address and alter gameplay constants, for an obvious reason—they are *constants*, which means they are loaded with the map, which is probably hardcoded. Still, I hope there is a way to find a workaround this limitation, and allow map makers to alter some of them in game. Stuff such as Upkeep Food Levels or Hero Attributes—Defense Bonus per Agility Point.
3. Increase the list of options for Hero Attributes
I don't see why we shouldn't be able to allow Strength to give Mana Regeneration points, or Intelligence to increase the Speed of the Hero (or even better—the XP Acquisition Rate), or even have the Primary Attribute award additional bonus depending on the type of that attribute.
4. While we're on the topic—allow the creation of new Gameplay Constants/Interface variables of certain types
Want a new Hero Attribute? Done. Want a new attack type? A new unit type? A new defense type? I know the WorldEdit/Warcraft engine is old and most of those constants are hardcoded, but if possible, those will be more than "neat little additions" that just make the work easier for some of the map makers out there. Those would be game-changers. If such changes are to be implemented, they will prolong the game and increase the modding appeal of the game.
5. Allow changing the cliff types/water type
Now this might be outside of the realm of the possible, but having the ability to use 2 City Cliffs in a map, or a City Cliff and the Outland Cliff, or the Outland Cliff with water would be great. Keep in mind, I'm not suggesting having more than 2 cliffs per map, just changing the types of those cliffs. This could be implemented through "new" tilesets, for all I care, that are just combinations of existing cliffs.
Additionally, allowing to change the water type (e.g. Barrens using Northrend water, or Dalaran Ruins using Sunken Ruins one) would also be great. Taking the idea even further—a "white" version of the Outland water that can be recolored to fit the needs of the tileset would also feel great.