Something like http://www.mscsoftware.com/product/adams would be incredibly useful and probably much easier to build than FAE and other simulators.
The ideal for me would be something where we could build a mechanical design, setup sensors and actuators, link that stuff to a controller simulation, press a button and have it work. It'd be insanely awesome if you could do that and have a reasonable expectation that whatever was built out of it would actually work as simulated without much tuning.