Well here's how it breaks down (or how I break it down. Granted, I know I'm looking for patterns where there aren't any. And I'm fudging some information. So sue me.)
Each new game in the series has added (3rd parties aside) 3 new series: A modern one, a retro one, and a WTF one. Melee: Fire Emblem, Ice Climbers, Game and Watch. Brawl: Pikmin, Kid Icarus, R.O.B.
Now as for 3rd parties, 2 were added in Brawl; one being an iconic character and one being a favor for a friend of Sakurai's. If you're looking for nonexistant patterns, as I am, then that would seem to mean that 2 more 3rd parties will be introduced in SSB4; again, one will be iconic, one will be more of a favor for a friend.
These nonexistant patterns would suggest that in SSB4, as far as new series go, we will see:
1 (one) modern/popular series.
1 (one) retro/oldskül series.
1 (one) WTF/nintendohistory character.
1 (one) popular/iconic character.
1 (one) favor character.
My assumptions that would fulfill this nonexistant pattern would be that we will see the following characters in SSB4:
Shulk (probably is a better choice, can't think of anyone though.)
Balloon Fighter (Takamaru is a close second, but considering they were both in Nintendoland they're on equal ground there, but Balloon Fighter was originally planned for Melee/Brawl but got beat out by Ice Climbers/Pit.)
Duck Hunt Dog (Popularized the zapper, and would be a lively addition on par with G&W and R.O.B.)
Megaman (popular and iconic, highly requested character. This generation's Sonic.)
Pac-Man (kinda a stretch on the nonexistant pattern, but could be seen as a favor to Namco.)
Discuss, flame, masturbate, whatever tickles your fancy.