If one calls things like guildwars a "mmo", then MMOs are certainly possible.
You can have multiple maps that can retrieve the same set of data so the limit of 256x256 is not a problem. It just means in order to enter a new zone you have to quit the current game, and load a new map. Parties can either be formed randomly through people hosting and taking who ever comes, or from chat channels and forums and such.
This means that any sort of "MMO" will rely on having several things, the most important being having a large player base, so that when a player hosts " X MMO Y Boss run" it fills up fast enough, and so that there is a community to foster the feeling of MMO.
The main hindrance to sc2 is the lack of a secure data storage system, since any data files are stored on the persons computer and can only be obfuscated, meaning once people figure out your system cheating will be rampant unless blizzard provides a away to lock data.
Aside from that the limits are almost endless for MMOs given you can create a player base. You can run tournaments, have pvp maps, trading between players, dungeon/boss maps as well as more general open world maps. You can take that aren't found in other MMOs, but are found in wc3 custom maps. You could have an MMO with tower defence aspects, or even a more generic mmo with a tower defence mission.