Nothin.
Also I'll say right now that the novelty of "YOU CAN HAVE X CHARACTER FIGHT Y CHARACTER" in crossover games, has long worn out for me. Fights mean nothing if there's no story behind them. Some characters, like Shulk, can't even be well represented in Smash because their character is just too complex and they don't have a distinct personality gimmick, leading to Smash having a bastardisation of the character anyway. I'd be more excited about "everyone is here" if everyone was going to actually interact. But so far, they won't.
I have to draw out the one central sentence to the point here:
Fights mean nothing if there's no story behind them.
Not wrong. But Smash has never provided that story for me - now or ever. Little nods like Snake's codecs, Palutena's guidance, even story/adventure modes, all add spice at best.
The one time that Smash had a cohesive narrative was SSE, and look how that went down. A couple cliques of heroes formed, and eventually all went off to fight a small clique of villains, and then a big bad, while wandering around a handful of worlds, killing a half dozen bosses, and watching ROB's race go extinct. And it was fun, but we got bare glimpses and minimal interactions between the cast, who in the best cases segmented off into their own groups and couldn't widely interact, and in the worst cases had minimal to no interactions at all. ROB, Lucas, and arguably Ganondorf were the only characters who really got their own narrative arcs; the rest of the story was told by retreading familiar notes from their own stories (Red goes out and catches 'mons, DK's off to fetch his stolen banana hoard, How Samus Got Her Groove Back), or seeing what would happen if you sat the characters next to each other (blue-haired swordies join up with much rounder swordie to go whoop a bunch of ass, Weegee gets a face full of pow pow, a group of five protagonists go off to do protagonist stuff, almost everyone I haven't mentioned who showed up in a cutscene somewhere), and a couple just got the 13 inch purple rubber shaft (Tink, Wolf, Jiggs, Sonic, Bowser to some extent).
They may have seasoned it better, but it was almost all just more spice.
And for the most part, I see no issue keeping it that way.
The only narratives that truly make any difference to me with regards to my Smash play are the narratives that form around my Smash play. They involve the people I play with and the things we do.
More in-game interactions would just be more spice.
Again, this is just a matter of knowing what I came for, and crossover fic wasn't it.
Now that said, this next part does provide insight into why I think we, specifically, diverge somewhat on the point:
So because of that, the character roster was the least of my concern with this game. Smash Wii U's biggest failings were it's shoddy online, lack of varied content, and even it's mechanics to a degree. Ultimate's online experience is going to be shoddy still because a Smash game will never have dedicated servers (and rightly so) and the Mario Tennis demo proved to me that most Switch players still have really slow internet, so that's a bust. We're still in the dark on content. And while some mechanics changes are for the better, some look really eyebrow raising.
I play a fair bit of online Smash, yes, but I also play a fair bit of it in person. And of the two, in-person's always been the superior option, for a laundry list of reasons that could start and end at mediocre internet speeds but thanks to For Glory deigned to keep going anyway.
If your experience is largely online and you don't see the service improving drastically - and we're in agreement there, I'm afraid - the narratives you would craft around your Smash play would be
vastly different to those I'd craft around mine. If they weren't as positive, or as numerous, or as ingrained, then they wouldn't be enough to carry it for you.
I don't really have a solution for that, unfortunately. With minimal exception, core content is more likely to be geared towards filling a void in the core gameplay mechanics - there's a reason they just call it Smash and give it half the UI billing in Wii U/3DS, that's where the game's full focus truly is - than a narrative or feature void, and I believe they're making the right call in doing so.