Imagine Sakurai comes to you asking for input on the last dlc character. He'll put them in if you do a good job convincing him. What are the top five reasons you'd give to justify your single, most-wanted character? Sell your pick.
An advance wars rep (Louie for a while, but I fear Nintendo would just make him a bland Olimar clone) like Andy or Sami!
1) Series is important to Nintendo (it persuaded IS to release FE here, a smash success nowadays)
2) Series is important to Smash (We all know how well FE is repped in Smash, and it's very possible that had FE not been localized, it would have seen a much lesser Smash impact or even cancelation)
3) Series has been inactive for some time now (the last installment was on the DS, the same year as Brawl), and a rep would be an awesome way to rekindle interest in the series should they reboot it
4) Any rep could be interesting- a character using modern weaponry and guns? a character who's a non-anime human? a character who screams AMERICA? perhaps even a mech or tank user? check all. AW has a bunch of unique and dynamic systems in check to keep things fresh, so it's not even like a surge of AW reps could be boring and recieve "tank man bad" ala FE now
5) It'd be awesome, and smash is doing a criminal disservice by not including any maps or music from the games.
If they ain't gonna make PM great again, at least create a new AW game, @IS!
Edit: Andy is basically the figurehead for the series (Famicom Wars ignored), and Sami was planned for Project M, so why I named those two in particular. Any CO could work really well as a fighter- just let them use attacks based off units they as COs specialize in. Use a CO power as an FS, or if there's more AW reps in the future maybe even do some weird Dual Strike thing for doubles! The possibilities are very high, and I do believe many are sleeping on a potential AW reps' "cool" factor, even if they're extremely unlikely at this point.