If it was for all of Nintendo, including the maximum possible amount of series would be the priority over including multiple characters from one series. It is not about all of Nintendo (at least, the roster isn't), it's about the Nintendo characters that merit inclusion, which is often multiple from one series while none from another.
Plus Melee was also considered a "Nintendo All-Star" game. It even said so in the opening.
So how come Isaac, Starfy, Ray, Andy, etc. didn't make it into Brawl (like 3B said)? They all had games either on the Gamecube, the GBA, or both, and their games were either bigger than Xenoblade, they had more games than Xenoblade, or both.
For that matter, if being on the Wii is the only prerequisite for inclusion, does that mean we'll see characters such as Captain Rainbow, Zip (from FlingSmash), and Ray (from Disaster: Day of Crisis) in SSB4?
I await further holes in your logic...
Marth was not the "Japanese Retro character". He was added because he was the most popular lord and the face of his series. In Melee's development SNES wasn't retro. I know that flies in the face of your roster inclusion beliefs, but it only goes to show the recentness mean very little when compared to other factors. Like Marth, Isaac and Saki are the most popular characters in their series, and the face of their series, and will be included over Matthew/Isa, just like Marth was included over every other FE Lord, including the recent ones (Roy was added later).