Note: So I started this post months ago and never finished it, but I still think it's true and some of my predictions have even come to fruition, so I figured I might as well brain dump the rest and let you read it.
This is a thought I have had for a while now that was increasingly reinforced with each tracked part of my body. Obviously hands have made a huge difference. The first time I had my hips tracked with RIPMotion I felt even more grounded than before. The thing that solidified it for me though was when I was first trying out ElectricNightOwl's ArmSwinger locomotion demo.
The demo was really good and felt comfortable to use, but there was an area that he had created with uneven ground off in the corner that IMMEDIATELY got me feeling sick. Having put myself through all kinds of uncomfortable locomotion in the past this both surprised and intrigued me. As I played with it more I realized the issue was that my height was changing based on a ray cast down to the ground from my head, but that this was nowhere near where my brain thought my feet were going to be.
Some time later I was listening to Voices of VR #402 when Jason Jerald mentioned trying to skin a potato with one hand versus two. It hit me that this was essentially the same issue and that I finally now really understood proprioception. The different parts of your body have spent so long working in conjunction that when there's a mismatch your brain can't reconcile the data. Having my head as a reference point gave me an automatic assumption to where the ground would be, but when I put my foot there, the connection between my foot and head wasn't what was expected so barf.
Mel Slater talks about virtual body ownership, or being able to see your body, and how much that can increase immersion. In the case of that demo there was also no representation of my legs, but I suspect that would have only made things worse. It's well known that if the body is too mismatched from your own it will break immersion.
In other words, having your visual, vestibular and proprioceptive senses all convinced of the same thing seems to be essentially necessary.
Now, I don't think that simply having your body tracked is the solution alone, but without it the software cannot exist. Simply having tracked feet alone surely would not have solved my issue with that uneven terrain, but it would allow some exploration of haptic retargeting (i.e. moving the virtual floor to match the actual floor as your foot approached).
To further enforce this, every improvement in what we are tracking so far has been a drastic improvement.
As I said earlier, hands are huge. Touch release is only going to reinforce this. (Now that this has happened, I certainly am not hearing complaints about it other than tracking...)
Crude limb tracking is the next step. (Which now seems to have people excited as seen by CloudGate Studio's Fullbody Awareness Experiments). Some more on this topic was covered in Voices of VR #462 with Denny Unger of Cloudhead Games.
There would also be other benefits besides the tool to fight sim-sickness.
I recall Norm mentioning on This Is Only A Test (sorry, I forget which episode) about using tracked hips as a way to create an accurate hitbox for FPS type games.
Watching people in third person is much more compelling if it's an entire person and not just a head and hands, or a head and hands dragging around a limp body. IK solutions like Final IK are helpful, but not as good as it could be.
Now that we've skipped forward in time a bit from when I first started this post, we've got tracking pucks coming to SteamVR with people already strapping them to everything. It's an exciting period for VR and I think with these improvements it's only a matter of time before we can make sim-sickness a thing of the past.
Hopefully if you've made it this far I sparked some idea in you that would allow you to be the person to solve it.