Forgive me for making such a long bump, but I've had this idea for several months now and I've /always/ wanted to see this implemented in a Sonic game (a 2D game at the very least). Initially it was in the vein of having secret exits that unlocked new acts in the main zone a la NSMB -so it doesn't exactly fit this idea- but then I've been thinking of it going the full mile and having secret bosses that took Sonic to a different path and to different zones -which fits this idea like a glove and is similar to the aforementioned Star Fox example. I even entertained the idea of adapting it to a structure somewhat similar to Sonic Adventure and having each playable character (let's hypothetically say Sonic, Tails, and Knuckles, are playable at the start) and they all start out at different parts of one island. All of the playable characters can easily work their way towards a certain goal (say, Eggman's base located at the center of the island), which can be done in about six "main" zones that the player can take. But the level design / game structure allows for characters to find a secret goal (or secret boss) somewhere in what would be the third (or second) (standard) act of the zone, which takes them to another character's path, and the player can thus continue their playthrough from there (or hop to another path if the player knew where to go, and so on). Then go the extra mile and make use of cast speciation like Palas mentioned, so like Sonic 3 & Knuckles there are areas or paths in the level design that one character can't access, but another character can. (I.e. An act within forest zone that has Sonic as the standard character you play through, but you have an cave covered with felled trees that Knuckles can destroy; or a temple that is separated from the main path by a large body of water that Tails can fly over to). Maybe go another extra mile and have "bonus"/"secret" zones that aren't part of the main campaign for the characters. And if one wanted to, go an even further extra mile and have areas that can only be accessed through use of powerups (I.e. an area in a temple level that has torch holders that open a sealed door-naturally, they would be lit by players who have the Flame Shield). The replay value would be absolutely insane, to say the least, and all of the above doesn't even cover the idea of alternate/side missions that don't involve the player doing the main objective (some of which were mentioned by Palas -completing a level in a set amount of time- or -as some pointed out- done in Shadow the Hedgehog).