Its an educated guess that PH won't leave the site derelict and do nothing. The masterplans are laid out and principles agreed. What I cant say is whether an approach will make any difference, or how much that may cost.
There are other issues as well, which would have an effect on any potential buildings on that site, and that's the tunnel that runs near directly below it and is why there is a section of the Liverpool waters that seem 'flatter' than the others while on plan. That's something that only extensive surveys would tell us, done in consideration with what form the building above it would take. The forces of a Stadium would be very different to the forces of an office building, or several on the same footprint. I don't know that answer to that though.