There was that sticky 3D physics thread and it had some examples. And we have these "little game templates". Also "useful code snippets" ...
Maybe, to begin with, could be a 3D Sokoban. Only blocks / cubes on a 2D grid. The game logic would be 2D, but it could be done with 3D collision detection.
Maybe a 10 by 10 grid. Than a Low-Poly 3D character. Standing, Walking, Pushing a block, maybe a "winning"-animation. There are Blocks for the wall, a box for pushing and targets. That's it.
And from that, we could go on. Climbing and doing ladders on some blocks. Changing some blocks for "better" visuals like corners and other walls. Adding diamonds to collect. Adding more animations for the player. Adding "monsters".
After some tweaking it could be a Tomb Raider from 1997. We would have to do some rules for sizes, or object style, etc. I would use the FPSC Classic "grid", but splitting it up for boxes in the "grid".
Level-Structure could be "text" at first. One letter is one element. Floor is everywhere the same. Later we could change the level-save-format.