I don't tend to have time to play many computer games these days (though I made exceptions for Portal and Lost:Via Domus), but I still like to keep abreast of how technology trends change in games.
A concept that I think is interesting in games is to "raise the stakes" as the game progresses, so that later levels seem to be more important than earlier ones. A couple of contemporary games (neither of which I have played) that embody this principle are Katamari Damacy and Spore. As the game progresses, the very scale of the action increases, causing the early parts of the game to pale in comparison.
3D graphics have become much more impressive in the past few years but the trouble with 3D is that the extra degrees of freedom make the controls more complicated and difficult to learn (especially for people who don't play many 3D games).
I think it would be interesting to apply the "increasing scale" concept to a 2D game using modern graphical techniques. My idea is a 2D-scrolling shoot-em-up. You start off with a very small, simple "asteroids" spaceship with a very simple weapon. As you blow things up, you can collect more weapons which make your spaceship bigger. Using these bigger weapons, you can attack bigger and bigger targets. The view gradually "zooms out" as the size of the ship, weapons and targets increases.
This idea also reminds me of the Seven Force in Gunstar Heroes on the Megadrive. Easy parts of this boss appeared in an early level and then in a later level there was a "scaled out" version which was much harder and with smaller sprites.
And with modern technology, 2D games can be seriously beautiful. Aquaria does actually zoom in and out a bit, but not by orders of magnitude as I'm imagining.
[...] 2D game I’d like to write (which might even be the same game as that one) is a bouncing ball [...]