Primordial Desert
Try it. :p It has prebuilt pyramids with traps etc, too, so you could probably take a look at those and work out how they work.
Seems interesting, but it also seems like too much desert and wavyness. I prefer to keep it to a good seed (probably something from this
link; Some sound promising enough). Plus, I want the pyramid you start outside (and may need to solve an introductory puzzle to get in) to be big and somewhat complex inside. I mean, I want it big enough that you may need to make a stop or 2 to the top in order to trigger another/next series of traps/levers. The rest of the dungeon will be deep underground with bits of natural light at times, and uses some glowstone or torches (I may look into researching some cool lighting trigger effect tricks using falling gravel and/or glowstone).
I was thinking we can make use of some tricks and traps and counter-measures that require knowing binary to solve (Like placing torches in the slots that equal let's say '13'. 001101, so torches in slots 3-4-6 and then pull the lever opens the next door, or something like that. Similar can be done with minecarts and rails and pits with pressure switches (and maybe other minecart tricks).
There will be places that have no lighting, and you may need to use the steel+flint to be able to see (torches are pretty much only for puzzles, and be sure you kept them all). Some underground lanterns may be available (fueled by netherrack, and sometimes rigged with/near wood, and even if you try to disarm it by punching out the wood (also a rule not to do: mine), there may be sand that falls and collapses the room, or fill it with water or magma), and some will become traps. If you accidentally burn down wooden support beams, then a cave system will collapse (using some falling sand/gravel tricks I thought up that can be used). You'd have to be cautious; especially if you're playing on any difficulty besides peaceful (suggested for first-timers).
We can also make use of ice and heat traps and whatnot to trigger water flows as well. Basically another goal of this is to think outside the box and put out all the stops in creative trapping and puzzling, and maybe add a jump puzzle here and there. And who knows? Maybe a stray treasure chest will contain some old TNT to solve some puzzles, or skip some puzzles. But beware which ones you skip, they may prevent you from finishing the tomb and escaping with your life.
A pretty cool cinematic trick to signal an end or end of the beginning or something should be a minecart rollercoaster that runs over a redstoned plate as it's moving quickly (like the wooden switch), TNT is detonated (or a chain-reaction of arrowshots) chase you down while the ride is caving in. Other redstone switches are triggering and changing your course slightly (but still essentially railroaded towards the end) while all this is going on. We'll need someone who knows how to set it all up however, along with a launch system to keep the minecart moving from beginning to end (with maybe some jumps).
I would imagine the best way to get the final product done and made will be importing individual elements or sections (say, chunk-based scaling (What is that? 32x32xinfinite?) elements). That sort of tiling can make developing the infrastructure and overall building of the final product easier. Depending on the biomes (much exploring initially, of course) we can probably clean out the tomb area, fill in the infrastructure, and then when its time to call the project final, fill in the rest of the gaps with the infrastructure, double-check the design elements, then bury it all under sand and a layer of dirt or whatever the prior biome was. I dunno. What do you think?
When I have the time, I'll probably spend some of it making a demo tomb with a few simple puzzles and tricks and traps. Get a feel for what I'm aiming for design/element-wise above and underground. Heck, even an obelisk can lead to a hint, you just have to get to it from inside and take a good look.