How about a glass-bottomed infinity pool spanning the 417 between the buildings? I know that it is a very long span (>130 metres), but the weight can be supported by cable stays back to the buildings – like a suspended bridge. There could be a higher pedestrian crossing to hold the tops of the buildings apart, if there was any risk of them tipping together.
OK, getting serious for a minute; the 417 still needs to be widened through that area. That will require new ramps from/to the 416 as they, I believe, only have room for three lanes under their bridges. This might mean that the interchange can be reconfigured – but I doubt that the MTO has the stomach for that. I suspect that there will simply be bridge replacements, with wider, longer spans. This will be disruptive.
Also, according to the last MTO plan that I saw, the Richmond Road (SB) to 417 (EB) circular ramp is to be removed, since that lane is needed for the widening under Richmond Road. Instead, traffic from Richmond Road will be routed north on Holly Acres to a new 417 (EB) on-ramp. That new ramp will be a continuation of the existing Holly Acres off-ramp.
Also, I expect that the Richmond Road (SB) to 417 (WB) ramp will also disappear. As it is currently constructed, it limits the roadway to only three lanes. If the Jersey Barrier were removed, there would already be the needed four lanes. (Remember, that Jersey Barrier was only recently added to prevent people entering the 417 from skipping across to the ramp to the 416.) Again, Richmond Road traffic destined for the 417 would be routed to Holly Acres.
I suspect that the Richmond Road (NB) to 417 (EB) ramp will also be closed, to remove the weaving that happens in that auxiliary lane.
This is a big change, and the MTO is planning on making a lot of roadway improvements (read: WIDENING) to the south end of Holly Acres. The intersection with Richmond Road will need to be enlarged, with more storage space. There will, likely, be a third lane added heading north on Holly Acres; while south-bound will, likely, remain as two lanes between the off-ramp and Richmond Road.
It is unlikely that the MTO would allow a driveway to be added to Holly Acres in that area.
Oh, and did I mention that Graham’s Creek flows through that triangle of land?
I like the idea of twin towers that incorporate a 417 pedestrian crossing, but I suspect that there are a lot of other open parcels of land that could be developed for a lower price.