Don’t burn me alive, but I think Waluigi has a very solid chance.
A.) Chrom is confirmed to still appear in Robin’s FS despite being his own fighter, showing that characters can appear in both a playable and nonplayable capacity.
B.) Waluigi (for better or for worse) has been one of the most, if the the most talked about character throughout the speculation season. If Nintendo wasn’t already aware of his popularity, they certianly are now. Which leads to my next point:
C.) Waluigi will sell. Among the casual Smash fanbase, Waluigi is a very, very popular canidate, and him finally making it in the game will get people talking. He’s not a niche choice, he’s a recurring character that appears in some of Nintendo’s most well known games. Nintendo is picking the choices themselves, and I find it hard to believe they would pass up the huge oppurtunity to maximize sales.