if the DLC was chosen after E3, Waluigi must have definitely been CONSIDERED at least. remember that Reggie said that he would inform Sakurai about the demand for Waluigi, and then a few months later Sakurai said Nintendo co-picked the characters. however, this doesn't mean Waluigi is definite. Sakurai said Nintendo would give ideas for characters and he would choose them based on potential moveset. now, while a lot of great fan-made Waluigi movesets exist out there, and Sakurai has managed to make unique movesets for characters with seemingly little moveset potential like Duck Hunt, Wii Fit Trainer and Pac-Man, it's not definite that Sakurai could come up with Waluigi moveset. I think his most likely case would be if he's a Isabelle-sorta character, whether to Wario, the Mario Bros. or someone else entirely I'm not sure. another thing to mention is Piranha Plant. while we all love Waluigi's son (or is that daughter?) and are very proud of him, he could be a big blow to Waluigi's chances as DLC, emphasis on COULD. Piranha Plant is very alike to Waluigi in some ways, he's a weird, jokey Mario character and I couldn't imagine Sakurai including two similar characters from the same series as DLC - well, until Corrin came about (see, he was good for something!). Corrin, while having some unique qualities, such as his drill-thing and dragon transformation (the latter of which is probably someone's fetish, but I won't shame, my fetishes are way weirder), is at the end of the day, still a sword-wielding lord with a counter, just like Roy - even then, Corrin could've been a one-off, since it was the Smash team who came up with the idea, not Sakurai himself. over all, if DLC was chosen after E3, Waluigi's DLC chances are 50/50 if you ask me.
if the DLC was chosen before E3, then that'd be bad news. Waluigi is unpopular in Japan (at least in comparison to here), and Sakurai could've easily misinterpreted the Western love for Waluigi as a fandom in-joke akin to Shrek, the Home Depot or Reckless Safety Notice Man (which is to be completely honest, understandable), hence his "just because you try hard doesn't mean you'll make it into battle" pic of the day. Waluigi has about a 10% chance of making it to DLC at max if DLC was chosen prior to E3.