It came up in another topic, but I think the idea is very fun to think about and to share.
Basically, I would love to see a lot of large, pre-existing lists/groups of people, objects, creatures, or whatever to be able to be used for P, PO or PAO systems (or similar). If you happen to know large groups usable for systems, do post them!
Periodic table of the elements.
The table houses 118 elements, of which most can be turned to or seen as a certain object (like a balloon for helium and the Hindenburg airship for hydrogen)
A list often mentioned here consisting of 802 numbered pokemon, and also a lot of unlisted pokemons and/or alternate pokemon forms. Most are very distinguishable by looks, though some are rather similar. Outside of the pokemon themselves, the series also houses dozens of human characters that can be used.
A cartoon similar to pokemon, with also a lot of notable creatures and humans. While I don’t have an exact number, it does run into the many hundreds of creatures.
Smaller series than the previously mentioned, but still with a notably sized and numbered list of robots, of which most are very distinguishable. Besides that, it also has a few dozen notable humans.
Game of Thrones cast
The Game of Thrones has seen many characters come and go, with dozens having a part in multiple episodes.
The Walking Dead cast
Similar to the previously mentioned, The Walking Dead also has lost and gained numerous characters over the years, making for a large cast.
Star Wars species
As a large, detailed universe, the Star Wars series have a lot of species, of which only a few ever made it to the big screens.
Star Trek races
Just like star wars, star trek too has gained a lot of variety in species and races over the years.