Its already been stated fighters have to come with a stage, music, and Spirit board.
You are shooting for the moon if you are expecting Waluigi at this point.
It's hypothetical, but Waluigi Pinball does exist, it's not like we didn't get a 1st party DLC who got both a stage and a spirit board.
Same with Geno (he's Square tho), Super Mario RPG is yet to get a proper representation in Smash.
And if you're familiar with the Mario spin-offs, you would realize the potential that Waluigi has in terms of soundtracks, and spirits.
Mario Tennis on 64, the game where Waluigi made his debut, is still not represented in anyway in Smash, that game has one of the most badass Mario soundtracks in the whole franchise (Game Point 1 & 2, Tie Break, and Tournament Round 2), give it a listen sometime.
And then you've got the human Mario characters from the N64 Tennis and Golf entries, those could act as spirits since they're from Camelot, same as Waluigi.
If not, remember there's always Mario Party 3, Waluigi was a big deal back then, kicking Bowser's ass and taking his place as the final boss of that game, with Waluigi Island acting as the final board for that game instead of the usual Bowser's Seat Burning Lava Resort.
And if Waluigi is the Mario spin-off rep, then Geno is the Mario RPG rep. So you got Forest Maze or Bowser's Castle with the chandeliers as a stage, cool, you've got plenty of spirits to choose from, you don't even need to fill the spirit board, 7 is the minimum as Banjo's spirit board showcased.
So what about the soundtrack?
Well I think it's fair game for Geno to have new remixes from across the entire Mario RPG series, and not just Super Mario RPG.
Geno was in the very first installment, and was a big part of it. Not to mention that, the original Paper Mario was actually called Super Mario RPG 2 during development, so even if Geno was absent, his essence was still there.
If Geno's circumstances were better, he'd be the Sheik of the Mario cast, he could've made it to Smash (which is still possible) with only one appearance in his series of origin, and maybe get to appear in spin-offs like Mario Kart, same way Sheik is in Hyrule Warriors.
Btw has anyone played Mario Kart Tour?
Doesn’t anyone found it sus that not only Waluigi Pinball returned as a race track, but it also got majorly updated in the looks department? It's like they made the track from the ground up. Other tracks I tried only had minimal difference to accommodate the jump to mobile devices, but I don't know, Waluigi Pinball in MKT received a brand new paint job, it's like it was remade to be used as reference for a grander game like Smash.
It wouldn’t be the first time they included something from the mobile iterations, Fire Emblem Heroes received a new remix in the base game, so maybe...