As most of us have already discussed, Sakurai does not have many characters to choose from left. While some people like to think that Sakurai is the type who is going to cut half of the roster just so he can add more characters, the truth is he likes adding more variety. There is enough evidence out there to show us that King DDD was cut in Melee for the sake of having more characters (Ganondorf, Young Link, Roy, Pichu, Dr. Mario, Falco). Waddle Dee is a safe gamble for Sakurai that has more upside then down. Think about it, very few people I've met actually dislike him as a character, the only real complaint is that he is a generic enemy. or that Kirby as a series is finished. Still, most people seem to think he can translate to Super Smash Bros well; and he can. Sakurai cares as much about variety in playstyle as much as he does the individual character itself. Waddle Dee has that. Heck, Kirby's Return to Dreamland already gives Sakurai a lot to work with, especially when you consider that a majority of the attacks for Metaknight and King DDD in that game came from Brawl. There is already a starting point for Sakurai to work with.
Yes, popularity could be a factor, but at this point it is not as though adding Waddle Dee is going to prevent someone important. The only thing that might hinder a Waddle Dee appearance is Sakurai fearing showing favoritism, but since he hasn't handled the Kirby series in a while, I think his (and his team) focus will be on the Kid Icarus content, where as HAL will probably spear head (no pun intended) much of the Kirby content from Epic Yarn/Return to Dreamland.
tl;dr version: Sakurai likes to gamble, and Waddle Dee is the safest gamble with the best upside. Brings a fresh moveset, from a popular series, from a recent game that outperformed Skyward Sword in Japan, and is a likeable character. My gut tells me Sakurai will make this move.
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