What usually happens to me is that, after a while, I recognize the well that I keep returning to and then decide to totally do it differently. There are certain shapes/scenarios you keep returning to, and it is hard to break loose from them. But recognizing them is one step towards changing things up. One thing I did was change my style: I was a very careful mapper, thinking about the placement of every object in the level. Then I decided, for a new mod, to do the placement of objects more off the cuff, and instead place the enemies more careful. So one way to get inspiration is to change your approach a bit.
You can get inspiration from other mappers. I usually get this by playing them and then see what makes a particular level work. For instance, AReyeP usually builds his level around scenarios. He takes some of the coding features and builds scenarios around them. For instance, in EoD you had a couple of scenarios where you had to take enemies out using the fireballs. Stuff like that. So perhaps a new approach for you could be to think in terms of scenarios and not be too concerned about making the level feel like a real place.
You can work with coding features to spice things up. When I mod, I usually have one mod in the works that is quite coding-heavy, and another which is lighter on coding or sometimes it's just vanilla Wolf3D. This requires different approaches to mapping, and then one style can inform the other.
Finally, there are times when I'm really stuck. And usually I just go do something else and then come back to a map. But occasionally I watch playthroughs by Balames87 to get some inspiration. He has played a lot and his commentary also shines a light on how players experience a map and what works for them.
Oh, and it's also worth playing shitty mods because they teach you what not to do. Or they use elements that are interesting but are used badly. Perhaps a different modder can improve such stuff.