I spoke to eyebo online earlier this week (before his retirement), asking him what dimensions he had in mind. I don't remember the exact numbers, but what he had planned was two layers (15 in height) and one space in between all macroblocks for fitting. He had also considered three or four layers, but agreed with my concerns about four layers possibly being too flat and thus perhaps limiting the builders ability to stand out from each others.
I support Cxom's 75-player three-layer design , which looks very promising. I also agree with Osaka's latest post - with the enormous cluttering of blocks this track will be, having the lights on is critical to avoid driving in pitch black shadows for most of the time.
Another idea me and eyebo were pondering was vertical transitions. This would mean that one driver would end his part by propelling the car upwards into the next layer, in which the second builder have started by catching that car (most probably using the upper part of a loop) - or vice versa by letting the car drop into the lower levels. This idea was founded in the discussions after completing D25, where people were suggesting the creation of a layered District track in Stadium.
This idea requires more, however, from both Cxom as the "main editor" (to use book/movie terms) vizualising where those would be, and from the two builders creating the parts. Especially the builder of the later part will have difficulties knowing how much his start will play out and the two builders should keep in close contact to ensure a smooth transition. My suggestion would be for the two builders to be each others beta-testers, which solves these problems by clear communication.