Everything humans create is an amalgamation of other's ideas we enjoyed in the past, take pride that your work is a love letter to creators who came before you feeling the exact same way you do!

Or a mode where you try to navigate the levels with a restricted or harder moveset due to playing one of the npcs/enemies.

Maybe a sharp change, like if the main game features many puzzles make a fast paced sonic style mode or vice-versa. Or more strange mix-ups like the Mean-Bean machine fight. You could also make a mode focused on defeating every enemy in a level including hard to find enemies.