Smourshing!
I found some of what I was looking for in Smash 4, just not quite in the way I expected it.
Single-player content, that is. The lack of an adventure/campaign mode still bothers me a bit, but once I started referring to the "challenges" screen for activities, I actually felt like Smash for Wii U was trying to give me things to do. The activities are diverse, covering almost all of the game's numerous modes, and all of them can be completed as a single player.
Here, again, is where my concerns about the disorganized menus come up. As used-to-it as I've become, it's still a dumb hassle to have to back out of three levels of menu, check the challenges screen, then dive three more menus deep again just to get to the next thing. (And did I mention that, incomprehensibly, the menu buttons are not touch-enabled on the Gamepad screen!?)
That aside, the game is finally feeling "real" to me with these challenges. Tasks for different modes have given me a finer appreciation for some of them - like the pachinko-styled Trophy Rush, and the Angry Birds-esque Target Blast - and character-specific challenges have at least started to familiarize me with some of the new fighters. (I still don't understand Villager or Wii Fit Trainer, like, at all, though.)
I've also worked my way through the rest of the game's Events, some of which were ... just ... unbelievably maddening. I haven't sworn at my TV this much since I was a teenager. Not really sure if that's a good thing.
Progress: 76/140 challenges complete, 227 trophies, finished all Solo Events on Normal.