The ability to customize or toggle additional drag features such as lift of a wing, hull shape affecting max speed in water, and customize friction as a value rather than by texture material.
Support for scripted displacement on the map. E.g… scripted weather event triggering dynamic waves in a large body of water. Possibly the hull or moving object creating wake from behind.
I realize these are mostly air and nautical related asks but I feel that area was lacking in what could be done in gmod.