I had a lot of fun with SEUCK back in the day, and 3D construction kit (even though it as horribly slow). Oddly, it seemed the C64 version of SEUCK was actually better. Notice the sprites are still the C64 24x24 size even on the Amiga! Weird. Oh and the Custom Level designer on Tower of Babel was also awesome. I always wanted to get my hands on the developer suite they used for Damocles/Mercenary III.
Other than that... learn to program! It's the only way for ultimate freedom, of course, but it does take some work. I should probably make the Mr Beanbag engine into some sort of kit!
EDIT: I found "Reality" a bit of a disappointment, to be honest. It seemed like pretty much all you could do was pick a sort of game out of three and then change the graphics.
