Dunedain19

Whenever I get that I play through some Thief/TDM FM's and I find the inspiration starts comes back rather quickly

You could always decide what kind of room you want to build and then just go for it. It may take more tidying up but it'll most likely be more innovative than it would have been if you bogged yourself down with planning every inch out before you started. It's kinda like when drawing: don't focus on the overall drawing of the room, focus on its constituent parts one by one and accommodate any flare you pick up along the way. That's how you develop a style!

What I do in that case is to stop mapping until the itching in my fingers becomes unbearable.

If you still want to increase your skills during such phases, I suggest trying to get into other TDM related tasks. Shaders, scripting, etc...

There is much to learn.