One thing I highly doubt has been suggested, but would really really really like to see added, would be to make it so the "choose a level" thing after clearing the game in singleplayer only shows levels one actually beat. Like hidden levels in singleplayer accessed via a warp in a level, or something else, which should only be accessible after actually beating it.
Edit: I'm not talking about the level select feature in the secrets menu, just in case anyone's wondering.
If that reason is really lame, just take a look at the Mystic Realm (in 2.0, as there is no official 2.1 port at the moment). After beating the campaign, one can snatch the Chaos Emeralds without having to find the Mystic Temples, and even play unlockable levels without unlocking them, without using multiplayer nor the console, even play Sonic-specific levels as Tails and Knuckles.
Since that reason is really good, but a pretty rare thing, I'll also state that I'm working on a thing with different levels for different characters, and I don't want a player to be able to beat the game as character A, then access character B's levels through that save file.
And as far as I'm aware, ForceCharacter in level headers makes levels unselectable in the choose a level thing, even with the correct character. Plus I'd like to avoid ForceCharacter, for the sake of co-op.
How that would work with existing save files, I don't know, but I know 2.1 doesn't like 2.0 save files, so if you'll ever make save files obsolete again, I'd like to see this suggestion added.