I've built a fairly efficient gold farm above the Nether roof. It just requires me to idle near the build limit and the stockpile of gold nuggets (and rotten flesh) accumulates below.
Still, a fairly efficient production of gold nuggets means a couple hours to build up two and a half stacks of gold blocks for a beacon. And I could spend the time more productively than through idling in a closed box.
After sifting through possible small afk farm options to add, with most either requiring water or just not needed (e.g. due to having a superior full auto option in the overworld) I came up with one reasonable option - a sheep (wool) farm.
There's one neat design that keeps the sheep circulating in a closed path over grass blocks using entity cramming, artificial light and spare grass blocks outside sheep-accessible area should keep the grass regrowing. I should be able to have the sheep travel upwards using stairs instead of using bubble elevators. A dispenser with a long timer to replace worn shears, a window in the fences to shear passing sheep through, a hopper to collect the wool (and possibly not picked shears), and this seems like a concept that could work.
Still, one caveat I can foresee is that zombie pigmen may not take well to my attempts at shearing them - and the grass being one of few possible spawning spaces in the area they would definitely spawn given the opportunity. So, first, how can I spawn-proof the grass so that zombie pigmen won't spawn? In particular, so that grass can regrow and sheep can still graze on it? (and baby sheep can spawn when sheep are bred... I'm really in no mood to transport more than 2 sheep through the 1x1 hole in the bedrock.)
Also, what other caveats to look for? For example, spawn-proofing hoppers, droppers, other redstone, or watching out for other blocks being spawnable (stairs, fences?)