How about a WilderSpawner(™️) which can cycle through mobs on a daily basis, which also needs to get fed 1 Netherite block per week in order to keep it running? Additionally you won't know which mob type it'll be spawing the next day. It should change the mob type each day and not weekly so that the players aren't going to avoid feeding their WilderSpawner when they learn what mob type it's gonna be. 

Using the WilderSpawner will be a real challenge for the player to handle it correctly as some hostile mobs have to get treated differently in a grinder setup based on the type of mob. (could be realized using redstone, which switches the funneling system so that the mobs will be transported to a different killing chamber).

Mobs that it could be cycling through when fed and on cooldown:
Spiders
Cave Spiders
Skeletons
Zombies
Zombified Piglins
Sheep
Cows
Creepers
Drowned
Strays
Witches
Slimes
Illagers
(Please no Evokers, as i hate Vexes )

Mobs that it'll spawn if not fed:
Pigs