Is it possible to change a level's state by beating a different level and/or making progress?

[Biscuit]

To clarify: Is it possible to add/remove things in a level with events from different levels across the board?
For example: Adding Toad NPCs in a Toad-Town when you make progress throughout the game or adding a Yoshi in Yoshi's house when you beat a certain castle?

[deice]

To clarify: Is it possible to add/remove things in a level [...] from different levels across the board?

yes

with events

nope. at least not directly. you could have the trigger be an event, but the actual heavy lifting requires some lua, namely writing progress into the savedata table. then, inside the levels you want to change, you'd run a check inside the "onStart" event and show any hidden layers or do whatever if the values you're looking for are present.