>>724899141
I think you'd have to design a level so that it teases content that might be there, but you run the risk of blueballing players by overdoing it. Pic related spawned so many rumors in the 90s of being able to get behind Bill's house, because the overhead view never shows you just how far back it goes. What if it's a path? Bill is a notable NPC (one of the few with a name), he's the creator of the PC storage system, he turned himself into a Pokémon, there's a parody of The Fly and even has unique graphics, you can see Vaporeon, Jolteon, and Flareon via his PC, you're required to interact with him in the story to progress, and he even has official art. So of course the idea that his house could lead somewhere special was more believable than some random unnamed NPC's house.
Banjo-Kazooie had Stop 'n Swop, it teased the Ice Key before you even got 100%. You were left wondering how to reach it, how to collect these items, what they would unlock in the sequel.
Put in little off-beaten paths, like maybe you clip through a wall in one spot and find a secret path leading to an Easter egg. Or if you have a jungle level, maybe model what appears to be an area behind the impassible plant life and foliage. Make the world seem bigger than it actually is. More than having your level be floating in a void, have hard-to-climb sides that enclose it with invisible walls above it. Add occasional odd textures that seem to imply a greater story or the presence of something else, like a crater with an old pair of burned shoes in the middle. Make some curious things interactable to raise more questions. Don't overdo it, or players will start to think there's no substance, and be ready to build upon what you've hinted at in future material while still having some stuff lead to results in the current work.
