Thanks! I am also a huge fan of the Thief games (and the mod scene for them).
You've got some good ideas. I've thought about doing multiple levels; mainly unsure about how separate the levels should be. If they are totally separate (enemies can't go back and forth) then there isn't much point, other than giving the player some extra routes. If they're just layers of a level (upstairs, downstairs, basement) with free movement between then it is a bit more difficult to present clearly. (Let's say an enemy is spotting you from an upper balcony; how would that be represented?) It might be worth it to have layers though. I've also done sketching on isometric tilesets; still not sure about that. It does make things less clear.
More types of levels is definitely a good area to explore. Getting more varied texture to levels, too; it's just a collection of connected rooms right now.
We'll see! I'm a professional game developer in addition to a hobby game developer. I don't always have a lot of free time. Once a year I take a week to work on things like this though.