Vanguard is not a possibility - they have a fortress in Malvern, they're not moving. Verizon? Sure, why not, it's logical. Would Verizon sell the land to a developer knowing they plan to move their empire here? Or is Comcast involved somehow...they do need ALOT more space.
The stock exchange is an interesting possibility. USAirways? I'm not so sure. They're headquartered in Tempe, but it's clear they view Philadelphia as their international gateway. I could see them move some operations to Philadelphia, but not the whole corporation.
If I'm WSC, I'm targeting banking/financial services. If only Lincoln would have held out a few more months...
I will say this - we (meaning the city) have a wonderful opportunity in front of us. If the right players are involved in the ACC, we'll have a positive domino effect that will last for decades.