We've all played through the Subspace Emissary and had fun with it, right? I did too, but unfortunate as it may be, it seemed to be lacking in a lot of areas. Granted, it was a very good first step in that right direction (in terms of bringing all these characters together for a story setting), but it was only a baby step. Personally, I feel that the only redeeming factor of this mode was the cutscenes, but even that could have done a lot better. So as good as this adventure mode was at first, here's to what could make it better next time:
Rule #1: NO. SUBSPACE. GARBAGE.
Honestly? Abominations that look like they belong in their own dang series? Way to shaft other Nintendo enemy characters that could have been. Is it really too much to ask for the biggest Nintendo villains around (Bowser, Ganondorf, Medusa, King K. Rool, and other enemy forces from different series, Space Pirates from Metroid, etc etc....Eh, Bowser may turn good at some point later into it) to come together to bring the world down using all their resources without all that made-up Tabuu junk and stuff getting in the way? If anything, Master Hand and Crazy Hand should be the only Smash-exclusive elements to it.
The world map. So could have been some sort of artificial Nintendo World with the most notable locales of each series plastered throughout (Peach's Castle, DK Island, Hyrule's kingdom, etc etc), but SSE just had to waste that on a dry, generic-looking place. This is one thing where I liked Melee's adventure mode better.
There could be a few simple places thrown in; if they are generic (city, plain, etc.), they should be provided with that gritty Nintendo feel and refer to some series (a lot of them for the price of one simple place). For instance, if there's a nondescript volcano somewhere in there, it could be populated by Gorons (Zelda) and whatever-other-fiery/rocky-species-from-another-series. It gives that versatile flavor that it should have.
More supporting characters or NPCs. C'mon, one or two that barely made a difference in SSE? Think of it this way: the main characters are essentially the peak of a mountain, and the supporting cast would make up the base of the mountain. And if you've got too scarce an amount of supporting characters, that's no good for that mountain, being all peak! Not cool!
Now there would be many of these NPCs to pick out from several Nintendo series, each with varying roles in another SSB adventure mode's story. Could be either worked into gameplay (with certain tasks that they may ask of you to fulfill) or cutscenes devices. Some could be some sort of shopkeepers, but for what, I'll get to that later.
Now, I'm not saying SSE was bad (and it was good in its ways). But some things in it were quite off and I'm just saying about what could make another Smash Bros. adventure mode better. You've got to consider that not everything Nintendo has done is 100% perfect/god-like and that there's always room for improvement. And that's when you should try to think about it.
These are just the basics for now, I'll get more specific about these in subsequent posts here.