I know just how you feel...
Illhaven, Invasion, One Day, Strange World, Shrakith'a...I must have played all of the good ones except Hollow (because I'm stuck on the third map), Xerania (because I can't get into the final area), and Vigil 99 (because I can't get it to run in Unreal Gold). I see exactly what you mean.
I'm trying to teach myself UnrealEd so that I can make a Single Player map of my own, but it's pretty rocky for me, even with all the tutorials on the internet. However, if I persevere, I can put a concept I've come up with, known to me as "The Gallery of Nesmeynom," into .unr format. With current real life pressures, I might not get done at all, but this is a goal that I have in mind.
I'm also trying to decide whether I want to experiment with mapping or scripting first, but that's another story.
If you also have UT, you might want to check out Legacy too. It's an excellent Single Player experience.