Rumors are flying around the Smash Bros. community on forums regarding the final character to be revealed in the fighters pass.
Before jumping into these rumors, I must warn you that nothing in this article is officially confirmed by Nintendo, this is just what the current 'rumors' are. According to a 4Chan post by Leakguypossibly (shady name, I know), who claims a source has told him that there is going to be a bonus echo fighter for Joker, this fighter would be Kasumi from Persona 5. Leakguypossibly also says that Nintendo is planning on giving her a proper reveal and that she would be released after Persona 5 Royal releases in the west.
Moving onto Waluigi now. Brian David Gilbert recently made a video dedicated to asking Nintendo about creating a Waluigi video game. Funnily enough, Doug Bowser, now the Nintendo CEO, replied to the Twitter video with a clapping emoji and a purple heart. Smash fans are speculating that this approval towards Waluigi could lead to a new game for him or even an announcement from Nintendo about him being included in the fighters pass.

It should be noted that Waluigi is already an assist trophy in Super Smash Bros. Ultimate, which really lessens his chances for being added as an official playable fighter. If I were to give my opinion on whether or not these 'rumors' hold any validity, I'd say they don't, and that Waluigi probably won't make it into Ultimate purely off the fact that he is already an assist trophy. As for Joker receiving an Echo fighter, this is more likely, and I can see Masahiro Sakurai and Nintendo giving players this echo fighter as a 'bonus.'