I am going to try something here. I want to make an adventure that culminates in discovering and destroying a yuan-ti temple. I am going to document my thoughts and progress here. And, because I know how I want the adventure to end (the temple), I am going to work backwards from there.
The highest ranking bad guy is a yuan-ti anathema (from Fiend Folio), though at CR 18 I doubt I will use it as written but, instead, utilize the idea. The anathema is a 25ft long giant snake body with a humanlike torso, two arms, and six snakes instead of a head. They are incredibly potent psionicists and very evil to boot. In my adventure, the anathema is worshiped as an incarnation of the god Merrshaulk.
Because of the tremendous power that could be attributable to the anathema, I am not sure if it is already present in the temple and being served by the yuan-ti worshipers or if, like in Red Hand of Doom, it is being summoned to this plane and the PCs arrive just in time for its incarnation.
Regardless, the actual big bad guy of the adventure is the high priest of the temple, a massive half-black dragon yuan-ti abomination with class levels (likely cleric but maybe some fighter-type levels thrown in). He wields a huge sword (either a great sword or falchion) and his symbol of office is a wide headdress made from the spines and skulls of his victims arranged peacock-style. Because of his size, the headdress is not unwieldy in normal wear (although it would be difficult to fight in it). This abomination has long been in my villain repertoire but he did not have a name yet. I am calling him Venkatesh (which is actually the name of a colorectal doctor where I live).
The rest of the temple is filled with the more exotic and snakelike yuan-ti cult members. The cultists that can pass for human are often lurking in the human world, accomplishing their leader's goals there. Some human stock are kept as slaves and sacrifices (perhaps even serving as food; have to check what yuan-ti eat).
The temple is going to be designed for snakelike creatures with architecture foreign to legs. When I first thought about this, I thought I was being clever in imagining ramps (with molded curves) and poles, both smooth and "branched", to serve in place of stairways. Then I read their Monster Manual entry and found out both were listed there. My legacy of independently having the same ideas as others lives on.
Another idea I had was to sprinkle traps throughout the temple that activate when stepped on. The yuan-ti, having snake bodies, would slither over the ground without setting off the trap (the snake's weight is distributed over the length of the body on the ground, so it neither pushes directly down on that one particular flagstone like a foot would nor does it exert a body weight's pressure while on the plate like a foot would). Intruders, however, would. This allows me to add traps that affect the players without breaking verisimilitude by leaving players wondering how the yuan-ti get anything done while avoiding all the traps.
That is it for now.