Here's you an idea (though some will complain about locked out levels)...
In each world, hide half-keys. Each save file randomly generates various half keys and you find these hidden within various levels. In your single save file, you'll never get a full key.
When you Streetpass someone, you'll get to "copy" one of their half-keys (similar to the Puzzle Swap mode). It could be half a key you need to complete a full key, or it could be half a key to a key you don't even have.
These keys will open gates in a "World 9" that allow you into brand new levels. I'm thinking, maybe one half-key per world (i.e.: 8 keys) and a total of, say, 12 levels?
I suppose you could also buy keys with PlayCoins...
It seems very Mario-themed and would be something I'd be in for (unlike, say, Street Fighter Figure battle... *yawn*...)