Morbi
Scavenger
You make a good case; however, I do not think many people even know that the franchise exists to be honest (including myself). It might have been pertinent to include the most important Advanced Wars character as an assist trophy in Brawl. I mean... look at how many people think Isacc is likely based on his inclusion as an assist trophy. If we were to be perfectly honest, most would not speculate in favor of characters like Saki and Isacc had they not been included in Brawl.But I think bringing new series will inject new blood to the game, you need to see the whole picture if you add another character from Fire Emblem it will bring his moveset and probably a part of the Fire Emblem but by now nothing more, instead if we bring another playable character from any other series that hasn't a playable character it could bring something completely new like new stages, new items, new assist trophies and aven less or more trophies, stickers and music.
To answer you answer about the Advance Wars series is by now one of the oldest and with many games under his belt without a playable character here is list of the following games:
Year ReleasedGame TitleSystem1988 Famicom Wars Family Computer
1991 Game Boy Wars Game Boy
1997 Game Boy Wars Turbo Game Boy
1998 Super Famicom Wars Super Famicom
1998 Game Boy Wars 2 Game Boy/Game Boy Color
2001 Game Boy Wars 3 Game Boy Color
2001 Advance Wars Game Boy Advance
2003 Advance Wars 2: Black Hole Rising Game Boy Advance
2005 Advance Wars: Dual Strike Nintendo DS
2005 Battalion Wars Nintendo GameCube
2007 Battalion Wars 2 Wii
2008 Advance Wars: Days of Ruin Nintendo DS
You can see that it has many entries and by now many characters, for me is a total blasphemy that there is no a playable character from this series, if it was with only a few games under his belt I can see the reason but now with more games than StarFox, Earthbound, F-Zero, Kid Icarus, Pikmin and other franchises it is now the time to include it by far the most underrated series in Smash with little or not at all mention of it.
Yeah but in the next installment of Smash and if there is a new Fire Emblem game should it have another character because is in another high-point and neglect once more the spot for a new franchise that could make it into Smash like Golden Sun, Xenoblade or Advance Wars even if by that time and even now they are worthy of a spot?
Xenoblade is the game where Shulk is the main protagonist and yeah is high selling game the bad thing it has not many copies available and are really pricy so it could be bigger and more known but for now is still a niche game.
.n_n.
There are not many other Nintendo franchises that NEED to be represented. We already have the essentials. It might be prudent to introduce some new franchise to aid their decadent sales though.