So I was watching some reactions to Smash Ultimate and all that, and something hit me. Something that gives me hope that we will have at least a semi-substantial Adventure /Story Mode component in the game.
When Sakurai is talking about how you'll start off with 12 characters and will unlock more fairly regularly, he uses the exact wording of:
"We've streamlined the conditions for unlocking new fighters, and we want to make the process interesting, so it feels like you're constantly recruiting popular characters to your side."
Maybe I'm grasping at straws, but the way he mentions "making the process interesting" and "recruiting characters to your side" makes it seem like there is gonna be an Adventure/Story Mode component where you travel through different levels and fight different characters, and beating them adds them to your roster. This is pretty similar to how Subspace Emissary worked, where you recruited characters to your roster, and those who weren't on the base roster for the rest of the game modes were also unlocked for those modes.
I could see some sort of Adventure Mode where you play as your Mii Fighter and you embark on a quest to defeat Master Hand or whatever, and you enlist the help of these Smash characters to do it, which would explain the wording of "recruiting characters to your side". So you get a bunch of sidescrolling action-platformer levels, with one of the fighters at the end being a boss. And of course you don't have to play as your Mii Fighter since you're recruiting these characters to your army, so you can play as them.
Of course alternative methods of unlocking characters such as Classic Mode or playing loads of VS matches would exist, but those wouldn't be streamlined OR interesting, as past games have proven. So yeah, I think there will be at least a 4-5 hour Adventure Mode where you unlock characters. It would also be a great way to make use of custom equipment (which I'm pretty sure are coming back given amiibo data is carried over).