I don't have any in-depth storyline ideas for you, though some of the most entertaining FPSC games I've played on these forums have been the Nazi Zombies Fan Maps. Small, confined buildings with multiple rooms, and endless zombies. Unlockable rooms with new weapons give the player something to look forward to, and a reason to keep playing.
Though, if your computer isn't the best (I know mine isn't), you can do a few things to increase FPS Creator's performance right now, which will most definately allow you to have much larger maps without a drop in performance.
PERFORMANCE TIPS 
1.
Grab an FPS Creator Mod (Fenix Mod and Project Green are both
free, so why not get one of them now?). Not only do they provide several new commands and features, but they
greatly increase your frame rates.
2.
Turn off dynamic lighting effects in the preferences tab. Yes, your guns won't light up the room around you when you fire them anymore, and you can't use flickering lights, but most FPSC games will run at nearly
twice their speed when you turn off dynamic lighting. And if you focus more on level design, and confrontations, the player won't really miss the bright muzzle flashes.
3.
Turn off shader effects under your preferences tab. Pretty much the same as #2. Shaders are great, but you don't have to have them in order to make an appealing game. And without them, your game will run faster.
4. It's already been said, but, use the least amount of dynamic entities you need in your levels. Weapons, doors, enemies, first aid items, etc. There really isn't a need to make 100 wooden crates interactive. Besides, dynamic objects don't recieve lightmapping from static lights. And proper lightmapping can really make a level stand out.
5.
Level breaks. If you've ever played the early Resident Evil games, you'd know that almost each and every individual room in those games required a seperate loading time. I wouldn't recommend going that strict with it, but you do have 50 levels to work with, so splitting up the floors in an apartment building with seperate levels will keep refreshing your game's performance.
Kravenwolf
