Long story short:

-2D platformer with very similar physics to Sonic the Hedgehog. Ideally, start out with a pre-constructed Sonic-style engine as the 'base', then expand/add some tricks for the player to use from there.

-Lots of curvy, loopy levels -- very little in the way of square blocks or running along straight-line surfaces.

-Large, non-linear world (a-la super metroid). Ideally, a seamless one -- where the game isn't separated into particular 'chunks' or levels, but the engine knows it can unload whatever's far enough offscreen so the game doesn't make the system chug.

From what I hear, something like Game Maker has me covered on all except the final point -- people have already made fairly good Sonic-the-Hedgehog-esque platformer physics engines for it, and it isn't expecting your level map to be composed of squares, but it likes it if you split things into 'level'-sized chunks, and if you don't it doesn't handle that well.

I'm wondering if Construct would be a better choice? My goal is to be able to jump in and prototype/world-build extensively without much in the way of custom coding/scripting on my part, and then once I've got a meaty game going, bring in / hire people for music/graphics/any necessary coding.