So what you’re suggesting, that users can just add Pokémon to the server to use on their teams even if only in formats such as Hackmons, is impossible because there will be trolls who use it to upload nsfw content and show it to kids. This is why uploading custom sprites itself isn’t implemented. Furthermore, this technically wouldn’t work either because for two people in a battle to be seeing the same thing, the information has to be provided by the central client, not only one person’s computer.
So what you’re suggesting really is that the community works together to create new Pokémon designs that are quality controlled to be up to the standards of Pokémon Showdown!, to use in a side format. This already exists, and is the Create-A-Pokémon, or CAP, subforum on Smogon. This format even has a ladder on the main server (CAP).
Otherwise if you just want Showdown to trawl through every ROM hack ever, datamine them for stats, and upload the huge amount of fanmade Pokémon, including new types, abilities, moves, sprites, models (because every Pokémon needs a sprite and a model now) then you don’t understand how insane an amount of work that would be. It already takes a fair bit to implement a new generation’s 100 or so Pokémon when the games come out, and that’s all hands on deck. This would take years for absolutely no pay off, since most fanmade Pokémon are bad or unbalanced.