Gameplay should matter most. Online is very desirable so you can fight opponents without always needing to have them actually over, but that comes second.
I am against a 'ranking' system in Smash or, at least, one that other players can see. It would make sense to match players against others with similar skill levels and I'm sure there is a way to implement that without having some sort of online public ranking. Also, this should be optional. You should be able to switch off the matchmaking system so that you can fight a specific person or people who might be way better or worse than yourself.
I could also see a system where players make a game with any name they choose, and the settings will be available to see for all players who enter the lobby to look for a game. Then, they join a game with settings they like, or they can automatically join games with certain settings.
Eh. A little rudeness couldn't hurt. My last post was less than charming. That said, I think I've used online mode maybe twice (to go up against other MYMers) and it seemed to work alright. Whereas I've accidentally started a match forgetting to change the match settings. . . . man, I can't even count. I'd say I have a dozen or so friends who play Smash (at varying levels of regularity) and none of them even slightly cares about online. To most people, Smash is something you do with friends. Not something you sit by yourself and play against anonymous strangers with. Personally, I would be totally fine if they just cut online altogether. Hence, the most important fix for me is those darn match settings.
It takes about 3 second to change the settings. Maybe 6 if you're not being very attentive and do it wrong. Granted, it SHOULD auto-save each time you set them, but this is more of an overlooked detail and not a glaring flaw like online play.
Perhaps the reason lots of people don't care about online play is that is was just so poorly implemented and laggy that it wasn't worth anybody's time. I myself played online a few times and I could see how having a good online system could encourage people to play (ie. more setting options and less lag). In Brawl, I could literally pick Ike, spam f-smash about a dozen times per minute (rest of the moves are jabs and grabs to un-stale the f-smash and get enemies to about 50% damage), and win all the time because the delays and lag didn't really give people any time to dodge or react to big powerful moves. Critical timing is the lifeblood of all the Smash games, and with 1-second lag it is just nigh unplayable. No wonder people never really liked playing online.