I love stuff like this- great article! I have often wondered how many various ideas/concepts surrounding skateboarding games have come and gone over the years. I would have loved to have seen this idea come to fruition as I played Skate or Die on Commodore 64 (yes, I'm that darn old). I often wondered why they never made a follow-up to 720, as that was a pretty popular arcade game.
Other thoughts along these lines: what was Neversoft working on after Proving Ground? I talked to the devs in-person at the THPG media event and they assured me they were working on some killer stuff for the next THPS title, but the plug got pulled on that next title as they shifted to 100% Guitar Hero development. I wonder if Black Box ever started dev for Skate 4? Same with another title for Grind Session or Thrasher Skate and Destroy. I bet there were multiple games that got dev started on them and they were never finished due to publishers pulling the plug on them. I would bet some really cool ideas were in those various games, but we'll likely never know...