This is already happening as we speak ... the X-plane default mesh (or the HD Mesh) do all use an irregular mesh .... which does exactly this: making less triangles where landscape is less interesting, and adding more, where more happens. But also remember, that triangles are not only needed for elevation but also to represent (reflect9 the landclass (as texturing is decided on per-triangle basis). In this old(er) interview, you can see quite a few screenshots, showing the irregular mesh (and other details about the scenery generation): http://www.flightsim.com/vbfs/content.php?13946-Interview-With-Andras-Fabian