Replayability game design challenge

Hi. This time am not looking to solve a bug or even talk about Unity at all (so am not sure if this is the site to post it in the first place), but instead talk and try to solve a game design problem.

See, I lack replayability in the levels of my game, is too linear and doesn’t last for long. I don’t really see this as a big problem, but I can understand why some people think it is. I have a few ideas on how to solve it, but my game is complicated to explain. The easiest comparison that comes to mind is Captain Toad Treasure Tracker: the levels are linear, short and small.

Simply making the level bigger is not feasible, since the camera generally needs to target the whole level at once, and adding more content is not an option, since I don’t have the resources and I know for sure that my game has ENOUGH content.

So how should I present it in order to gain replayability?

Again, I have certain limitations: the level can’t be too big, which obviously means that it can’t have several divergent paths either; doesn’t have to be linear, but given its small size it’s basically bound to be; I can’t (or should) increase the amount of content, and finally, procedural generation is not an option since the levels are handcrafted and really procedural stuff doesn’t improve quality.

Basically, if you had to develop a game similar to Captain Toad, how would you make it more replayable?

One solution (the only one that I have right now) is to focus more on bosses, since those aren’t limited by the size of the stage (but that doesn’t impact the levels themselves)

For some context, Captain Toad Treasure Tracker:

I really like how Downwell boosted its replayability by adding semi-random (every stage you pick one from a few randomly selected) abilities that alter the gameplay and introduce new strategies. Different combinations of abilities that you may accumulate allow for different combos. But since it’s semi-random, you cannot just pick your favorite combination of abilities, you have to play until you accumulate them. But by not being entirely random, the player doesn’t feel like he’s forced to do something specific or play in some way, there’s always some choice. Instead, players end up trying different playstyles every time.
They do similarly with weapons. Instead of letting the player find its favorite / optimal weapon, switching weapons is tied to getting other powerups (heal / more ammo), so it again doesn’t feel forced, but players will do it anyway and always play with different weapons (because the benefit is too good to lose).
Note that the combos I mentioned are not extra features added to the game, they just emerge from how you can effectively combine abilities.

I think this otherwise simple game is packed full of design gems that should be taught in schools.