I was thinking today that I may need to change my maps, npcs, quests and dialogue to be stored in the database rather than in XML files. Then I can just load the maps when players are currently on. This would allow for a much much larger world, without running out of memory. I could also increase the allowed amount of players as well, not that I think too many players will ever be a problem.
Then I will work on the map editor. It'll be a C# windows app. You'll just point it to the location of the database and it'll give you a list of existing maps or let you create a new one. Basically you'd choose a tile from a scrollable tile list, click on the map and set that tile's type.
In the future it can be expanded to do NPCs, dialogue and quests as well.