There are many other threads like this one, and I don't really like to be repetitive, so I'll just say...
Characters: I'm okay with any character's inclusion, just so long as there are no randoms, like Ghirahim, Demise, and Pac-Man. All I'm asking for are established characters whose inclusion would actually make sense and not be there simply as fanservice; also referred to as "OMG, it'd be so cool if...!" Though
a bit of fanservice wouldn't be too bad. I just don't want Nintendo to run the gambit of including characters that might come off as "clones" and wasted potential. There are many other negatives to this as well, but I'll keep this short.
Basic Options | Structure: In terms of options, I'd really like there to be more customization in battles
(*chuckles* as if there wasn't enough already). There should be more modes included, and better online play. It would also be a nice addition to include a sort of reward system, much like the one featured in
Melee. This was a feature omitted from
Brawl that I dearly missed. In
Melee, the player could earn various stats and such, and be rewarded points (sort of like style points), which would accumulate at the end of a given battle or Classic, Adventure, and All-Star modes. It kinda forced the player to improve his or her fighting prowess, especially seeing as how it penalized you for stuff like "stale moves." Those who'd prefer to play without it should be given the option to turn off the feature, or could simply ignore it - that is to say if such a feature is included in the new game. Aside from that, more collectibles, please.
lol
Stages: With regards to stages, I'm okay with whatever they decide, as SSB has never failed in offering the best stages. I just hope they stick with the best stages and ideas, and keep the fanservice to a minimum - all in good tastes.
In terms of ideas, I think it would be a neat idea to introduce the option to remove stage hazards. This would be a great addition to the series, particularly for purists who just want an all-out brawl without interruptions from a voracious shark popping up out of the water or a Bulbin blowing up the stage. This might also serve to further enhance the customization aspect that the
Smash Bros. series has come to be known for. In addition, it could push the envelope even farther by building on the aspect of changing the conditions of the stage, such as rain, snow, nighttime, and things of that nature. It would also not hurt anything if Stage Builder mode returns with more options and parts available, of course.
Gameplay mechanics | Physics: In terms of physics, I don't have much to say aside from "return to
Melee physics." Much of what I speak of is covered
here. That aside, it'd be neat if they built upon such mechanics as Super Smashes. There could be multiple smashes, team-based smashes, and event-specific smashes. The sky is the limit.
Welp...that's about it... :/