You know, i was thinking about what makes each possible Mario character more likely to be included than the others. So i looked at it a more in depth:
First Appearances:
Toad (Obviously older than the others)
Waluigi (2000, Mario Tennis)
Paper Mario (2001, Paper Mario)
Bowser Jr. (2002, Super Mario Sunshine)
So Toad has obviously been around the longest, while the other three follow each other very closely. Not that it matters, but the time they have been around may have something to do with it.
How about their roles or side appearances?
Waluigi has only appeared in spin-off games, appearing in Mario Party's, Mario Kart's, Mario Tennis/Golf and even Dance Dance Revolution. But never an actual Super Mario game. Toad and Bowser Jr have played roles in numerous Mario games. Toad is a main character in SMB2, but since then hasn't had a very exciting role. Bowser Jr. has been a side villain for the most part, helping his father take down his long time rival. Paper Mario is obviously the main character of his own franchise, which is given another installment once per console. (or spin-off. i consider it a bit of both)
Again, this may not matter, but Waluigi has the most appearances, but not in the most important of games. When i see people say he could represent the spin off games, i say guess what? So could everybody else, including everyone who is already in Smash Bros. Invalid argument if you ask me. Bowser Jr and Toad have seen a decline in "important" appearances as of late. Paper Mario has a game every few years. I don't know that there is any decisive answer to who would benefit from what here. Bowser Jr and Toad have kind of joined Waluigi in the spin-off boat.
Popularity:
I'm not entirely sure how each character fares, but Waluigi is very popular in Japan (While still maintaining a strong hate base as well), while Toad and Bowser Jr appear to be more popular in the West. Paper Mario doesn't have as strong a fanbase, but i wouldn't always say the character with the largest fan base will get in.
To prevent this from turning into a Wall 'o text, this will be the last thing for now. Moveset potential:
Waluigi could use stuff from the spinoffs. However, Peach already does this with her golf club and racket.
Paper Mario and Bowser Jr have imo, the best potential movesets out of the four. They have several items/attacks from the games they have starred in, and could be generally creative. (However i believe that there is a chance, however small, that they could be turned into clones)
Toad is a hard one for me. I don't see much in him. Peach already has the vegetable attack, and she uses Toad as one of her specials as well. To add, Toad's arms are ridiculously tiny, about the size Pikachu. Pikachu doesn't even use his arms for anything in his moveset, and i highly doubt Toad could realistically use them in a fight. I find that Toad's moveset would have to be mainly projectiles, which i wouldnt really care for. They may even have to edit Peach's moveset just to give him an attack.
Maybe it's just me, but these are some of the things i consider when discussing the Mario characters up for inclusion, if we even get one at all.