Hello Lads, I have been reading today and not really posting, trying to think of something useful to say with regard to this stadium idea. Although in theory the stacked stadium idea makes sense it is a preposterous idea that ultimately will never float.
I mean who wants to feel like they are going into the centre of the earth to watch a hockey game. The Diagram (well executed by the way) points to the fact that spectators would have long lengths to travel to get to their seats from the concourse (down down down) is what I see. If concerts were to be held in the arena portion, the logistics of moving equipment in and out would certainly be a challenge.
Ultimately, the design shown above would look MASSIVE and something that big would demand a stunning architectural exterior. Something incomparable to anything current stadium in the world. I don't think that we could truly justify a building of this type, it would just be too costly.
First and foremost, I think that we are getting ahead of ourselves here. Allow me to make a couple of points below.
Mega-multi-use stadiums just don't work. We certainly have the engineering capabilities to create it but it just doesn't work for creating dynamic (sorry) spaces. For example:
Any stadium with an athletic track immediately moves the spectators back from the playing surface. This means ideal seating for track events but not so much for sports (soccer, football, rugby etc). This is the reason why you often see the first 20 row of seats at stadia such as
Rogers Centre (the issue at Rogers it that it's the last of the cookie-cutters, and very much a baseball-first stadium, so any brand of football is going to be the poor cousin there [/digression]) and Commonwealth tarped over. This means that you lose the intimate feeling of being close to the action. I have watched soccer games in Italy and many of the stadia are like this it is a horrible fan experience.
Sure, a seating configuration that moves over the track is possible but this causes issues with the linkage between the concourse and the bleachers themselves. It just doesn't look pretty.
Hosting an event such as basketball, hockey, lacrosse in such a large space just doesn't work. The seating configuration is poor, there is no flow and there is just a lot of dead space. This can ruin an event. If you recall when the Raptors played at the Skydome:

This is an example of basketball at the Georgia Dome:
Great Football/Soccer stadia offer fans perfect site-lines and a tightness to the field of play. [my emphasis] This involves a field that is virtually right against the edges of the field with steep stands. A great example is Barcelona's Nou Camp Stadium:

This stadium's exterior will be undergoing a renovations by none other that Sir Norman Foster:
The ultimate hockey arenas also offer great site lines, an intimate feel with luxury boxes and solid acoustics.
EVERY stadium needs to have good flow to it, quick access to the concourse for food and beer and ample space in the concourse.
The best idea thus far has been two side-by-side buildings.