It would be great to have universal TWEEN function in expressions, perhaps with different options on easing in time.
output=tween(value1 to value 2, over amount of time in sec, type of easing)
walking from interior into exterior:
interior ->set brightness of interior TWEEN(100,60,2,0);
exterior ->set brightness of exterior TWEEN(180,100,2,0);
crossing the line between interior and exterior would result in dimming both - interior to very dark and exterior to normal in 2 seconds.
switching between music themes with crossfade
More examples can be described (cameras, visibility of objects, objects moves etc.), but I think it's sufficient.