Yeah, I've been there. Sometimes it can be that inner critic, nitpicking at story ideas. It's what causes me to stop and take a break for a while. Well, that and having to take a step back because I start spending too much time and mental energy on it, hehe!
I've bounced around a no-magic setting for a while; it's actually what I have been working on. It's different; in my mind it's more like a very very low tech sci-fi setting as opposed to a fantasy one (though there are elves, dwarves, etc.). For a time, I was working on a Redwall setting. The problem: I read only 2 of the books, and immensely enjoyed them. I didn't have the maps for the world, I really didn't know very many of the characters. I created my own parody of it, and while it worked there was an insane amount of scripting involved (lots of scenes, level-less, using DG for training skills with each NPC only training a couple of them). So, I went back to my original no-magic seems-fantasy-but-is-really-more-like-science-fiction setting.
Are you crazy for considering it? I don't think so. If you're burned out on a setting/idea, what are the odds you'll keep working on it? Then again, you have to make sure that you're happy with whatever choice you make, otherwise it's wasting time and effort. Going back and redoing everything is a pain, but if it means being able to have creative freedom then it I think it would be worth it.
On a side note... "Another AD&D mud??" I think the same could technically be said about WoT, Star Wars, and *insert any popular book, anime, series, etc* mud. I wouldn't worry about that too much. There might be a lot of AD&D muds, but how many of them would you say are good? No matter what theme you choose, if the content is good and entertaining your mud will draw people, I am sure.That was some advice a friend gave me; find a way to make your mud unique and it'll stick out in people's minds. There's one I used to play years ago that I sometimes wish I could play again, since it used some features I don't really see used anywhere.
Not sure if this helps; pretty late here, so I feel like my thoughts are rather ramble-y. No matter the choice you make, I hope it works out!