I remember in Flight Simulator, you could set for certain mechanical failures to occur during flight. I was thinking that a similar principal could be applied to NoLimits 2. See what happens if, say, a brake fails, a ride operator presses a button they shouldn't, etc. It would also be interesting to be able to design safety measures to be able to prevent a catastrophe in these types of situations. It would add a whole new level of realism to coaster designing. Also, destruction of the trains in disastrous events could come out of this. When testing an unfinished track, the train would fly off the end and hit the ground, shattering into millions of pieces. And it wouldn't look like crap, like in RCT3.
More dynamic statistics on display at the top of the screen would be nice. Currently, you have speed, forces, and framerate, and I think that's all. Height, distance traveled, roll angle, and others could be added.
I think an addition to the support system would be cool. To make for easier station and building making, there should be a kind of 'support' that when connected at four points creates a plane. This could be used to make walls, floors, roofs, and much more.
A more in-depth ride operator mode. I won't go into detail, but something more involved than just dispatching trains.