Let's first get dungeons working and worry about the rest later...
Petri ,
Because I am making level editor for my game and I know , is very hard to create a level editor and is harder when you have a game already completed and creating the level editor after that , I suggest you to create the level editor with the existing features that the game already have.
When someone creating a game , don't have in mind of creation of the level editor , because is more important the game being completed (That stage was completed) than creating a level editor. Some times when the game complete is not able to have a level editor. But each game or engine have created with some basic editor for levels or maps.
These forums helped me a lot , about my game and what problems I will have in the future. I am creating the level editor parallel with my game and when I have a new idea to add to my editor , I add it first in the game and then add (this idea only) in the editor before go to further developement.
About dungeon crawlers , which are years ago implemented already, I never seen any level editor in any Ishar or in any Might and Magic game or in any original Dungeon Master.
In fact none of the old dungeon crawlers ever had any level editor , and the Dungeon Master's newer remakes inspired to have a level editor and then the Dungeon Master Java.
After years when the technology was improved and more ideas added and the engines was changed then the level editors invented and this add longevity in the game's life.
None of the Westwood's Eye of the Beholder series their engine supported any kind of level editing ever, after making a long of research about level editing in Eye of the Beholder series since 1996 , I realized , I wasn't able to create any level editor in the westwood's original engines and I didn't have any access in their engine , so I decided to create my own engine from scratch what accept level editing capabilities during developement.
Even Dungeon Master community created a new Dungeon Master engines from scratch and Dungeon Master Java was a new engine.
So as you already have created your own engine from scratch and know its capabilities , you are the only who can create the level editor with the existing features.
So I suggest , to create the level editor with current engine features and if there are not modding abilities or new wallsets or new monsters there will be not a problem. The development takes a long time , all people here we are so excited with this game and this is the best dungeon crawler game ever created and people want a lots of things to be added and fast , most people here are not programmers and believe things can be made fast and easy. My opinion is to slowly-slowly make your work. You have the knowledge , and new knowledge will appear in time. (I have created 11 different engines until , arrive in the state I am now).
If I was you , my next step will be a basic level editor with current engine functions. And then I would released updates (Patches) of the game which will be parallel with the level editor and any kind of modding, or portraits, or monsters or any kind of new wishes people want.
Only YOU, are the Michelangelo of your creation , others are guests.