I think the best case for the roundhouse is definitely for at least the front 2/3 of it to be preserved and it serves as the front of a residential/hotel skyscraper. Unfortunately I don't think odds are too great for a best case scenario. I would imagine rehabbing the round house would be an very expensive undertaking and crafting a design that makes it all work and not look terrible also won't be cheap. I don't think the potential returns at this location would be strong enough to justify such a cost.
Maybe if Philly and penndot fixed the terrible mess that is the roadways in this immediate area it might make the area attractive enough to encourage a worthwhile development here and the massive parking lot to the Northwest.
This whole area is just a nightmare for pedestrians and cyclists. I don't think it would be THAT expensive or complicated to rework this area. The biggest problem is that some streets here are basically used as defacto highways, and everything is designed with the driver in mind, with big swooping ramps and interchanges so that drivers can go as fast as possible through here. Look at this map:
Red: This is the biggest problem with the area. So many drivers get off 676 then down 6th to get on the BFB. Instead of the ramp here coming down to street level, it should remain at the same grade as 676 and run directly beside 676 and right onto the BFB. All that traffic stays off the streets. The only downside is a wider overpass that peds have to walk under when going down 6th or 7th, but it's already a wide overpass, what's another 20 feet? Both should have a beautified pedestrian experience anyway. Also drivers will no longer be able to get off 676 and head straight down 6th street, which currently a lot of drivers do. But who cares, just get off one exit sooner and go south on 8th or stay on 676 to go south on 95. These minor inconveniences are more than worth it to keep all of this speeding traffic off the roads.
Green: Rather than these twisty highway ramps, just have vine street continue straight ahead 9th street and have the exit from 676 merge onto vine right after 9th street. The area to the North of this new vine street is now developable land. Also with the 676 to BFB exit reworked into overpass, there's now room for Vine street to continue even past 7th street and once again run along the north side of Franklin square. The walkway to the north of Franklin square becomes the southern sidewalk of this new stretch of vine and while not a ton of a room, I think there'd be room for some developable land on the north side of this new street.
Blue: There's just no need for drivers heading onto the BFB from Race to have this curved onramp. Just have them continue down Race and turn left onto the same ramp that drivers going up 5th drive onto. Make this ramp a couple of lanes wider to accommodate the new traffic. With all the changes that have been made, the approach to the BFB that currently houses the barren plaza that contains the kite sculpture is in much different shape. We lose land to the North with the new onramp from 676 and to the east with the widened onramp to accommodate race street traffic onto the 5th street ramp. But you also gain a lot of land to the southwest. Ultimately it's a much better public space because instead of being bisected by multiple onramps that you have walk past to access it, you just have the one onramp on its edge at 5th street and because of the significant decrease in traffic traveling around it.
Yellow: 5th street is a major bike lane and yet has anyone here ridden through the the 5th street tunnel? it's terrible. Cramped in this tiny dingy tunnel with cars flying down right beside you. Most cars heading north past the BFB avoid 5th anyway, there's really no need for cars to travel through this tunnel when they can easily take 3rd or 7th. Make the tunnel for cyclists and peds only and punch through the ceiling of the tunnel to open this up to natural light where possible.
Orange: If you want to go north on 7th off of Race, there's just no need for this dedicated street section to make it happen. Have cars keep heading east turn left right where everyone coming up 7th from the south drives.
black: these are all the streets and ramps that would be removed. I'd also liberally cut lanes around here, two on this section of 6th, at least one on this section of Race. Maybe a few other here and there.
If you do this, not only does it direct a ton of traffic off of these streets but it also greatly reduces all the breaks in the sidwalk around here. Plus in creates some developable land that could be sold to fund parts of this project and once developed would further improve the pedestrian experience here.
If something like this doesn't happen, I don't think there's any chance the roundhouse gets incorporated into a quality development here, or that we'd even get a quality development period.