I'm fine with the level selection for most series. I do kind of wish that we had more variation in Metroid stages, though. Most everything right now is either lava or acid (Frigate Orpheon, and to a lesser extent Pyrosphere and Brinstar Depths, are the only ones that don't feature lava/acid.) Same thing with Donkey Kong stages and jungles.
I also agree with everyone here that Bowser's Castle is long over due as a Smash stage.
Here's an actual character related question.... who can Nintendo add to give the final stage of the Wii U All Star mode some more characters?
The characters debut would have to be pre-1985, which leaves few option. Ice Climbers came out in January of '85, so they might be able to sneak in. Excite Bike and Balloon Fight were both released in '84, so they're good.
The only big third party I can think of is Bomberman, who debuted in '83.