I think Mewtwo fits in there incredibly well, making it a 48 slot/50 character roster. I honestly consider Mewtwo the most famous non-Mario Nintendo character not included in Smash 4 (given that almost everyone from Brawl returns). He'a the most requested character worldwide and Sakurai has shown interest in his re-addition. Mewtwo is one of the safest bets for a slot on the roster IMO.
Lucas doesn't really have to be there. Ness represents Mother alright on his own (not completely, but if we're talking about representing Nintendo best then Mother really doesn't need two characters, being a really small niche series anyway), being a good representative of the three Mother protagonists anyway. Ness and Lucas are both kids growing up, facing the hardships of their lives by overcoming their fears and experiencing disturbing and life-changing events while they mature. Lucas can warrant a spot on the roster, but he's not
extremely necessary to represent Mother in Smash.
And @
Spinosaurus
, you were saying that Palutena is he outlier in the sense that she doesn't represent a new IP, right? Rosalina and Greninja don't really do that either.
I'd say that those three are great additions due to their representation of series growth. Rosalina is one of the main supporting characters in Super Mario Galaxy, perhaps the most critically acclaimed Mario title of all time that revolutionized both the Super Mario franchise and the platforming genre as a whole. Greninja represents Generation 6, which follows a pattern of Pokemon representation but means a little more than simple tradition. Pokemon X and Y are revolutionary Pokemon games, introducing new ways to battle, easier training, amazing new Pokemon, and
walking diagonally. Gen. 6 obviously deserves to be represented now, but should be represented in future titles as well because of how much Pokemon has changed because of it. Palutena is a pretty tough one, but Kid Icarus Uprising was a pretty huge game. It was very successful, but it marked the first time that a series was reborn because of Super Smash Bros. Without Smash, Pit and Palutena would still be forgotten video game characters and Kid Icarus would be collecting dust in Nintendo's library of franchises. While Pit and Palutena work together to represent Kid Icarus's growth as a series (Pit with weapons, Palutena with powers), the main significance is that this is Super Smash Bros. Uprising is a huge success for Smash; SSB was 100% responsible for the game's existence and sales. Some may say that Kid Icarus is overrepresented as is, but given its significance and ties to Smash I can clearly see why there's so much content.
Wow I wrote more than I expected. So I guess those are my thoughts on series representation...