Stupid question: any chance that the parking garage that 33 Arch is cantilevered over could be torn down and replaced with underground parking and a decent-sized tower (in a decade or two, perhaps)?
So long as you have the money/its feasible to bury and build, and can get past the NIMBYS and neighbors losing their views, and ensure the wind from resulting from the close proximity of the two buildings doesn't turn the area into a perpetual vortex, why not? It'd have nice proportions, compared to our usually wide towers we've been getting, the accounting for the wind could lead to some interesting setbacks or some depth in the facade, and the general massing could play off the cantilever of 33 Arch.
I'd give it <10% chance.