-Brick reflection (you know when Lego bricks reflects stuff a little? Yeah basicly this)
-Water reflection and waves (we all know the water on the environment settings lacks waves)
-A way to prevent emitters/lights/rain/snow from going through bricks (suggested by Dante77)
-Hair customization (suggested by Trogtor)
Hmm, I the brick reflection seems interesting. But I don't think it would work well. It would make everything, even things meant to look like grass or something like that, look like plastic. Also, to make it look good, it would have to render the area a second time right? Sort of like shadows, but this would be more intensive because it would be in addition to shadows. I think it'd be better if this was an added colorfx, just to make it look kind of reflective, but really isn't. Same thing for water reflection, even if it seems like a good idea, I doubt it will happen.
As for the emmiters/precipitation
Doing this for real would require raycasting against bricks for every raindrop. It ends up being hundreds of raycasts per second and the performance impact is just too much. Raycasts shouldn't be that expensive but they are, and overhauling the raycast system is not a small task.
However, waves and hair customization seem like great ideas