The more I think about this, the less sense it makes. What's on the other side? Who walks over this bridge?
This is my major hang up with changes to Florida also. Where is the need?
Granted, you could create something phenomenal if money is no object.
Picture this, Florida goes underground between First and Fourth. There are underground access lanes for underground access to parking lots for both Memorial Stadium and Assembly Hall. The drives would connect to and expand truck loading bays at the NE corner of AH.
There would also be ramps down to multi level underground parking garages for the other three quadrants and perhaps under the current surface lot west of AH. Think Monroe Street garages in Chicago.
The bridge over Florida could be turned into a plaza of sorts, with seating and all the other special touches for the front of the new south end zone.
Take the surface lot west of AH and turn it into a multi sport athlete village and dorms of sorts. They would have all their own amenities and support including practice fields, courts, paintball, or whatever else it takes to get the big names here.
If you are going to dream, dream big!