Outside of Waluigi, who is the most likely Mario character to be included in Smash 6 in your opinion.
Hmmm... tough to say honestly. Despite the plethora of characters in Mario, if it came down to only 1 newcomer I don't see how it's not Waluigi. Now in the event that someone else does get in or if we simply get 2 Mario newcomers, I think there are some candidates that have serious pros and cons:
Toad: Toad is always an option. A huge part of Mario, one of the core members and is everywhere. From mainline games, to spin-offs, to marketings to TV shows, to the movie with a big role etc. It's just impossible to argue against Toad's significance in the franchise as a whole. The one sticking point is that... despite that he hasn't been added, despite being a sensible and logical addition already from
freaking Melee (like even that far back Toad could've gotten in and make sense), and so one has to wonder why add him now. Of course the same argument applies to Waluigi but the thing is... Toad doesn't feel like he has this outcry that Waluigi had, nor his popularity (I don't argue that Toad doesn't have fans, of course he has, it's just that there are characters who absolutely have more), and with Sakurai obviously taking that into account, I don't know if he'll consider Toad now.
Pauline: Pauline saw a pretty big revival in the Mario games after Odyssey obviously as the breakout star. Super Mario Odyssey was absolutely a big success and it even landed a stage in Ultimate with Pauline herself being included! After that Pauline has started making her debut in all kinds of spin-offs and is confirmed for the next Mario Kart, so it's safe to say she'll be a Mario spin-off mainstay going forward. There are definitely people who make comparisons between her circumstances with Rosalina's back in the day and that might translate to Smash, but I have a bit of a reservation. For one, even with Pauline having seen a big, big boost in popularity, it still feels like it didn't reach Rosalina's level (which is more of a testament to how popular the latter became) and Rosalina's popularity holds up damn well to this day even whereas I think after Odyssey, Pauline didn't quite reach those levels, and certainly not in terms of fan demand for Smash. However the big thing that makes the comparisons with Rosalina end in regards to Smash is that Rosalina had confirmedly a very unique concept as a fighter that covered an archetype not yet seen in Smash. Pauline doesn't have that uniqueness potential that Rosalina had, which makes her inclusion while still not impossible, at least content of doubt.
Toadette: I know, I know. Toadette getting in on her own may seem unrealistic at first and might genuinely not happen but, I would still like to talk about her. For one, Toadette has absolutely been boosted a ton in terms of significance and popularity. I mean, at this point she is a main character at least among the likes of Daisy, Wario, Waluigi and Rosalina and with 3/4 of them being in Smash, Toadette definitely has the content to back her up. Now why would she get in over Toad? Let alone Waluigi? If the possibility arises that Sakurai wants to add a Toad, why not find Toadette as the perfect vehicle to combine the attributes of the regular Toad, Captain Toad and the rest of the Toads along with her own little quirks for a fun moveset (or even play around with her transforming into Peachette? who knows...) That's definitely an out-there scenario but Sakurai has shown to be unpredictable at times so I wanted to at least entertain the idea. Why would she not get in over Toad? Well... simply because Toad is a far bigger character than her and if Sakurai wants to combine Captain Toad and Toad he could just do it with a normal Toad and see Toadette better fit as an alt. Out of the 3 this is definitely the wildest scenario but I wanted to at least entertain the idea of a left field inclusion and Toadette makes sense as a bit of a left field/surprise pick which also makes sense.
I know that the fandom speculates/wants characters like King Boo, Kamek, Paper Mario and Geno etc. but imo, these aren't happening for a variety of reasons, and seeing that the comment is already really long, I'll just say that outside of Geno, none of these have fan demand behind them, even with Geno, I can't see a world where, if Nintendo and Square negotiate for their characters' inclusions for Smash, they negotiate for Geno of all people when the company has so many bigger names. Paper Mario is a 2nd version of Mario that is actively going to take the spot from an original character just to represent a small niche that only a certain part of the fandom wants to give an unreasonably big spotlight and without the ease of development of clones, I don't see how the time and resources that go into making a unique character go into making a 2nd version of Mario. King Boo and Kamek are just names that some people in the fandom throw around but their best shot is getting in as a Piranha-Plant-style wild pick which I don't see happening for the Mario series 2 times in a row.