I had a huge problem with Melee's adventure mode.
It promised me a sidescrolling adventure.
I got a couple of stages thrown around a billion regular matches. And the only one I felt was worthwhile (though pretty darn good) was the Zelda one. The Mario stage was boring and straight forward, and the Metroid and F-Zero stages are just sad.
To me, SSE's only problems were the entirety of The Great Maze (forcing players to repeat stages with little to no differences as last time should be banned from video games forever) and having damage percentage, which works awkwardly on a side-scrolling setting and doesn't really make sense (you kinda only die because the camera locks up when you're hit, that's terribly lame.)
My dream Adventure mode is sort-of Melee-sized, but with a lot more side-scrolling thrown into it, and stages that actually worth playing in a way other than skipping everything until you reach the end. They could have a basic pattern for each stage that goes like, platforming stage -> character fight -> boss fight.
Let's take a Mega Man stage as an example.
It could be generally based on Wily Fortress stages from Mega Man games, start out in an outdoor area, beating up anything you find in the way until you reach the start of the fortress, once inside, some platforming challenges come up, like the classic dissapearing blocks, which would be a pretty darn good and fun challenge to any character that can't just stay floating in one place, and lots of classic Mega Man enemies along the way. Then you reach the typical boss door at the end of the stage, and inside, you have a pure Smash style fight against Mega Man.
Defeat Mega Man, and you move on to fight Dr. Wily, should be a pretty similar fight to the boss battles in Brawl.
And then you do the same thing for every obvious franchise that could have a stage until you have a mode that's long enough to be fun, but not too much so that you're actually encouraged to replay it with different characters without it getting tedious. They could make sure some characters have an easy time in some stages but having more trouble in others. They should try using a similar stage design philosophy from each franchise for the sidescroller parts, throw in a fight against a playable character, then a boss taken straight out of the current stage's franchise. Also, I'd love it if side scrolling sections had Stamina, but character and boss battles had Percentage.
Imagine this with five stages in a row, each one themed as one of the franchises in the game, and then a final stage with original elements from Smash Bros. series, with the Master and Crazy Hands as the final boss.
I'm not expecting anything close to this, but if it happened, I'd get like a heart attack or something, it'd be just crazy.