Wow. I am stoked. This game looks to be the shizNITE!
Online; For me personally, not having four player will be quite a blow. I always felt SSB games were a bit boring with only one on one. In fact, I remember getting the N64 version and playing it with a friend of mine for about 2 hours. After that we kinda sighed and said to each other, "Street Fighter?" "Street Fighter." The next day two of our other friends came over and they wanted to try out this new Mario VS Link game. We didnt stop playing until the next morning at 9am when one of them had to go to work. THEN we didnt mind playing one on one as much, because it was practice for when everyone came over.
But still, to me, SSB doesnt really shine until there are four people playing together.
3rd Party Characters; AWESOME. Gripe all you want, the more the merrier. I got over seeing characters together in games that made no sense for them to be together in a looong time ago. I guess I am the only person here who ever played X-Men VS Street Fighter, Marvel Super Heroes VS Street Fighter, etc. I mean, come on, Ryu VS Hulk? Yeah, RIGHT. No character is too odd ball for this game, ESPECIALLY if Snake is in it. Isnt he "Mr. Sneak around" guy? How opposite of SSB can that be?
If you want to complain about it because you were hoping for more Nintendo characters and you are afraid that doing this is going to reduce the amount of Nintendo characters you will see, then I have to say I understand where you are coming from and you are probably right. But you know what? I dont think thats gonna stop you from playing this game. In fact, think of all the Nintendo haters who look at that game and say "Aww its all Nintendo's stupid characters?! pfffft! Who gives a crap!" Then think about what they will say when they see their "boy" Solid Snake in there.
Other Characters;
I wouldnt mind seeing(3rd party only)
Simon Belmont(CastleVania)
Megaman(duh)
Viewtiful Joe(duh)
Rolento(Street Fighter Alpha 2&3)
Terry Bogard(Fatal Fury)
Leon(R.E.4)
Sonic(duh)
Raiden(Mortal Kombat)
Master Chief(Halo)
If Nintendo could actually pull off that many different characters from that many different games and companies, they would have practically something for everyone. Again, for anyone who actually thinks "What foo?! XXX wouldnt fit in SSB!" Remember this is a WISH list. This aint gonna happen. Also understand I can only watch Ken pummel on IronMan like he was a chump so many times before my whole thought proccess of "What foo?! XXX cant fight with XXX!" is completely shattered.
Because ultimately, I mean, you know, correct me if I'm wrong here, but... this IS a video game right? We are gods as far as video games are concerned and can do what ever we want with them right? So then, no character is to oddball to fit in any game. Hell! I'll add one more character! Dodge Viper(GranTurismo)
Hey! Does anyone remember that SSB knock off Konami did a while ago? I wonder if they could work it for Nintendo so Optimus Prime could be in SSB too? >=)
I also found Jump! Superstars to be one of the greater fighting games ever, as it was really just SSB with Shonen Jump characters. I wonder if Nintendo could get some of those characters in SSB? I know you will immediatly point out that most of those characters are already licensed to other companies (at least the ones that are out here in the US), but isnt that the whole point of the Snake in SSB thing? With that, I guess I need to add a few more characters to my wish list!

Naruto(duh)
Kakashi(Naruto)
Goku(DBZ)
Piccolo(DBZ)
Luffy(One Piece)
*Zoro(One Piece)
*sorry, I REFUSE to call him by his US changed name "Zolo". Thats just stupid.