I defy any company to make as much content and gamespace as they had in those old side-scrollers. Super Mario could afford to give players more content than they could handle. The core game was long and diverse, and then there was a massive side-helping of hidden stuff and bonus levels. 3D games don't do that. They can't afford to make half the content "optional," because then the games would be about two hours long.
huh? this is at odds with everything I've experienced of game evolution. Surely games are getting longer and more complex with the jump to 3D.
super mario had 32 levels, each taking a couple of minutes to run perfectly if you were a skilled player. mario 64 onwards.... well, http://www.mariomarathon.com/ you can see that 100%-ing the old games took about 1-2 hours. Mario 64 took 11 hours and sunshine and galaxy even longer (and I think that's based on starting from the partial play-throughs they already had in hand). Even just doing the bare minimum on 64 (without glitching it out to jump through walls), about 1/2 the availible content, took longer than 100%-ing any of the 2D games. 3D games like Mario, Zelda, GTA etc are CHOCK full of optional content and side quests, in the 3D marios you don't even have to visit every WORLD, let alone attempt every "level" (star, shine etc). Other genres, like jRPGs, have barely changed gameplay wise with the 2D/3D switch, with changes being largely cosmetic. And for every 3D game that forces you to explore every part of its small limited world, you can find an old 2D game that was equally linear.
as for the death of 2D, it hasn't died, just migrated. you don't NEED the full power of a PS3 or 360 to power a good 2D game, and you don't need the development team of a full AAA title and a price tag to match. Instead, 2D games are moving to handhelds, downloads and browser games. There are tons of great and *popular* 2D flash games, casual download games, handheld games and PSN/live games, they are just as mainstream as the big 3D AAA titles but because they are cheaper or found on more milited systems they are less part of the perceived market.
Maybe a worms style game with LBP physics and mechanisms would be good.