Since nobody else is responding, I guess I will throw in something, with the disclaimer that I have for some time been limited to APS format, but used to shoot film.
First of all, I wouldn't trade in the 50 even if you get a zoom that overlaps it, because it will still be faster and a little different.
If you are happy with the perspective of the 50 and not wishing for a really radical departure, but just for more field-of-view flexibility, I'd suggest the 24-70. A view wider than 24 can be great fun, but it becomes progressively so different in perspective and viewpoint that it requires a different way of looking at things.
In my current kit I have the 10-20 DX zoom (which would be equivalent to 15-30 in FX) and find it very nice to have, but I use it fairly seldom. It's a fine lens for traveling, where one often wants the kind of sweeping environmental view, but in normal use I find I use it far less than the normal and short-tele lenses.
This is largely a matter of personal taste, though, I think. I have always had a relative preference for normal perspective, and while I like the occasional venture into other viewpoints, I'm more likely to default to the mid-ranges.
If you could borrow or rent a very wide lens for a while, it might help to determine how much you really like the viewpoint.