I'm working on a 2.5D platformer prototype that aims for an open feel while maintaining familiar core mechanics.

Now, there's some obvious challenges with creating a non constricted feel in a spatially constricted environment. What I'm interested in, is examples of how game designers deal with the "here's a level, beat the bad guys/puzzles to get to the next level" design that seems so natural to most platformers (eg. Mario/Braid/Pid/Meat Boy to name a few).

Some ideas for achieving openness I've come across include:

• One obvious successful example is Terraria, which achieves openness simply through complexity and flexibility of the game-system
• Another example that comes to mind is Cave Story. Game is non-linear, offers multiple choices and side-stories
• Mario, Rayman and some other 'classics' with a top-down level selection. I actually really dislike this as it never did anything for me emotionally and just seems like a bit of a lazy way to do things.

Note: I've not actually had much experience with most of the 'classical' console platformers, apart from the obvious Marios/Zeldas/Metroids, since I've grown up on adventure games. By that I mean, it's entirely possible that I simply missed some games that solve the problem really well and are by some considered obvious 'classics'.