I don't think the Wario series should get another character. Besides the fact that Wario himself not being classified as a Mario character is a bit odd, I don't see what any of his Ware friends could bring to the table in terms of moveset potential. I'd love Captain Syrup to join, but let's be realistic.
And no, Waluigi doesn't count as a Wario character at all. They're paired together in the spinoffs, but that's it. Waluigi never appears in Wario's games. Kinda like how Birdo never appears in Yoshi's, or Daisy in Peach's.
When it comes to Mario subseries that should get another character, it's totally Luigi's Mansion. With just three entries, it's sold more copies than Wario's whole extensive collection of platformers and microgame collections. King Boo is particularly interesting, as he's a character with a unique body type, plus he is a ghost, which Smash doesn't have any of, and has a plethora of distinct abilities. He'd also bring in another villain, and a really famous one at that... the Boo species is one of the most popular and well known Mario enemies, after Goombas, Koopas and Piranha Plants. If not the ghostly king, it could also be Professor E. Gadd using his various gadgets... but I don't see that as very likely.