There's just been too many cases of less important characters from only one or a few more games getting in Smash for moveset potential or popularity for me to agree with your argument. Given what we have, I 100% stick to what I said. The recent Smash trailer for Robin and Lucina and the Gematsu Leak have only made me more sure of this. Palutena is the only character that's joined from an existing series that was the next more prominent character from the series, but you know what else she is, the next most popular too.
Impa's merits sound good on paper, but when you think about it, do you really think Sakurai would notice her? Recognize these merits? Have her planned to be added from the start?
Reflect off of what we have now, and does it still make sense to you?
Why would he do the same for Ghirahim? And he certainly didn't take any interest in Midna despite her being extremely popular. He was even hesitant to add Ganondorf in Melee because he wasn't sure if Ganondorf would appear in future games (and he didn't want to make Ganondorf a clone, though he did so by virtue of not being able to find anyone else to be a clone).
Also, what characters have appeared from an established franchise that have appeared over the course of several games that are one-offs? Retro characters tend to only have one game, and even then, the only character who fits the bill is Ice Climbers.
Sheik was included by nature of literally just being Zelda in a different costume. Sakurai thought it'd be a neat mechanic, and later he split Sheik off into her own character.
Robin/Lucina, again, from a franchise with a new cast every game.
Greninja? Pokemon will ALWAYS come back in future games. That's a given.
If Shulk gets in, its due to the nature of him being in a brand new franchise, and frankly he'd be the first and only character to date to get that kind of treatment (Pokemon had several games before Smash64).
For the other SmashU newcomers:
- Wii Fit and Wii Fit Resort
- Villager appears in all Animal Crossing games, that's 5 to date (the first was Japan-only).
- Palutena has been in all three Kid Icarus games.
- Little Mac has several Punch Out games.
- Rosalina has been in quite a few Mario spinoffs now and three main games (more than the likes of Waluigi).
- Same with Mii's.
- No need to bring up the third-parties at all.
Dunno what you're getting at.
Brawl:
- Pit is a retro character, but even then, he still had two games.
- ROB isn't an actually character, but he made numerous appearances in other games.
- Toon Link had Phantom Hourglass in development and Minish Cap already by this point.
- Diddy Kong, Dedede, Metaknight are obvious.
- Lucario had the Pokemon scenario going for him, he's guaranteed to come back in future games.
- Ike already had two games by this point.
- Zero Suit Samus appeared in several games already.
- Olimar had 2 games. Even then, its still a new franchise, so its a different situation with an established franchise.
- Wolf is a main Star Fox character.
- No need to bring up the third-parties, they're all well-established characters.
Lucas is the one exception in this game. And again, whole situation with the cast being all new in every game.
