I don't have any pictures unfortunately (the server I was playing on disappeared before I'd finished the build) but my favourite underground base design was an inverted glass pyramid. I'll try to explain it in a way that makes sense.
I quarried out a huge area all the way down to bedrock (49 chunks in a 7x7 square), and covered the ceiling in glass leaving a massive underground "hole". I then suspended my base from the ceiling in the centre of the area on three levels with glass walls that tapered inwards.
Level 1 was a 5x5 chunk "green" area that contained all of my environmental dabblings; it had a tree farm, a biomass/biofuel refinery and a laboratory for bees and trees. My magical workshop was also on this level. I had animals roaming around it and it was very free range and tranquil.
Level 2 was a 3x3 chunk "industrial" area suspended a good distance below level 1 and contained all of my heavy machinery. It housed my ore refinery network, my charcoal plant and my tritium generation facility as well as all of my item storage/distribution and workshops.
Level 3 was a single 15x15 chunk secured with warded stone and an MFFS system. It was home to my fusion generator and my matter fabricator.
I never really got chance to do anything with the exterior of the base site, and hadn't decided on a theme for the view outside the glass. I was toying with ideas such as recreating a Nether Fortress/stronghold or building a gigantic sculpture of a demon encased in ice or something. Sadly, that's when the server went splat and it was all for naught.
)