Yeah, I could get behind this with a little editing. I don't think we need to add that many mobs--we've got Husks already as a specialized hostile Desert mob, and still have all the other standard land-borne hostile mobs. Plus when Phantoms show up, that'll add to the variety there as well. But if we chose one hostile mob and one passive mob from your list, then I think that would be fine. I'd like to see a scorpion--maybe not even as a hostile mob but as a decorative mob (like bats), but it'd be interesting to see how they'd do the movement animation for a cobra. I think camels is ok, but a little redundant since we already have llamas. What would the camels offer that would be different from llamas? I'd be more interested in adding things to the game if they added to the diversity of the game.
Pyramids is a cool idea. I agree that they should be rare and need a map, since we already have Temples. Mummies spawning in a Pyramid is another cool idea--I definitely like it.
As for the Desert Storm... maybe that doesn't even need to be an event, but rather, it could just be a weather condition? Since it doesn't rain or snow in the Desert, a Sandstorm could be its equivalent? Sand gets through around through the air without affecting builds. Perhaps it could leave behind slabs of sand like how snow leaves slabs of snow, but you could shovel it up into sand blocks.
Anyway, I think this is a fun idea, it just needs some revising and, as X_a_ says, more detail.