I once worked on and off on a 3D engine in C++ for about 8 years and know how much effort and perseverance it takes. I was young and somehow lost track while hacking in memory pools, shader generators and a custom scripting language... Don't fall into this trap! Keep it simple, don't NIH too much and focus on gameplay!
One question I have: How do you generate those wonderful, non-repeating dirt textures displayed in one of your videos? Is it a procedural texture or is it based on marching cubes or...? They really look amazing!
Everything is based on math, so its easy to make things non-repeating. Voronoi Diagrams, "Lichtenberg" patterns (for the cracks, can't think of a better word), sin waves, random noise, simplex noise, etc.