Interesting article about the plans for revitalizing Smithfield and Wood, in part to capture spillover from Market Square (which does not have enough space to accommodate all the ground-floor establishments that would like to locate there):
http://www.post-gazette.com/local/city/2014/07/21/Planners-developers/stories/201407200210
As I noted in the comments, it is too bad Mellon Square has such a "look but don't touch" quality to it. In fact, I'd love to just walk through it sometimes on the way to or from my bus stop, but even that is difficult with the way it is laid out. And it looks to me like it is in fact underutilized, particularly in comparison to the revamped Market Square.
So my thinking is that it would be great if it was designated as an official food truck zone, which could transform it into a destination at least during lunch (and maybe some late evenings and weekends as well). To really make that work, I'd love to see the closure of much of the adjacent block of Oliver each midday. Combine trucks along there with maybe some more around the corner on William Penn Place, and I think you could have something really cool. And as noted, I think you could also look to do that on some evenings and weekends as well, particularly if Smithfield starts to take off as planned. And in fact it could even become a later-evening hot spot if done well.
Interestingly, this picture almost graphically demonstrates the concept:
You can actually see the people spilling out from the food trucks on William Penn Place--I think that was likely taken during the rededication ceremony, but I believe that should be a daily occurrence, and expanded if possible.