Of course, I understand that everything's still up-in-the-air at this point, but regardless, I'd be suprised to look at the roster and not see him there, similar to the situation with Ridley (but to an even greater extent). Next to Sonic, he has more support than pretty much any other character, and I'd even consider Geno a more valid candidate for a spot than him (though Sonic will be in a Mario cross-over game soon).
In regards to Wario and Diddy, there could be several different legitimate reasons as to why they didn't catch the fight in Melee. I'm going to go with either lack of time or lack of interest. Wario has only recently been acknowledged as his own compelling character, and, with all due respect, Diddy isn't the greenest patch of grass in the field.