What I do is make it so you have to play the game several times to get everything.Obviously you don't have to but there are those who absolutely have to find everything in a game so they will play it endlessly until they do.They make great beta testers.They will find any screw up for you.
So you go through the door on your left.It closes behind you and you can't go back through the door and see what you missed until next game.
Adding nifty things like deathtraps and things that explode if you get close enough to them (not all mines look like mines).
And if you want to be really sadistic you can add platforming to your game.
Jumping from point a to point b sucks in my opinion.It feels too unpredictable for me...but I got this thing where I like to see my feet when I jump but that's just me...
In case you haven't gotten it already download Nickydude's guide.
You'll need it.
There are great tutorials there,including quite a few scripting ones by Plystire...great tutorials.
I didn't realize until I saw those scripts of his exactly how much you were capable of doing with FPSC via scripting.I was impressed.
You might check out the Best of the Best thread in the Showcase forum.Playing those great games/demos will give you an idea of what you can do with FPSC.
At this point thanks to Lee,TGC and some forum members it is becoming clear that nothing is impossible in TGC.
RIP Floating Skull unknown-2007 we will miss you in FPSC x10