Top Ten Super Mario Characters Who Need Roles in Future Mainstream Games
This is a list of characters who should have been in the main games or featured in more mainstream games.
Waluigi was indeed born in spinoffs and has been in more and more spinoffs over and over. Seriously, he may have been created by another company, but he's not the only one who has. He's still a Mario character, and this guy has been neglected countless times. He has never been given a fair chance in any game where he could develop a proper character and a proper personality.
I mean, the guy has never even been in one Wario game! He's very popular, but this is only because of memes and his catchphrase, "WAH!" - obviously stolen from Wario. This character needs the proper development to become more likable, and the only way to do that is to give him a good role in an actual main game.

Daisy debuted in Super Mario Land in 1989, after few other appearances she disappeared for nine years until she was brought back in Mario Tennis (2000).
Daisy... read more
Princess Daisy debuted in one of the most successful games for the revolutionary Game Boy, Super Mario Land. So, she was born in a mainstream game. Her role was pretty much like Princess Peach's, as she was kidnapped by Tatanga, the alien. Not long after, she was forgotten for almost nine years until she made her reappearance in Mario Tennis. Ever since, she's been in spinoffs and crossovers to this day.
Now, she has developed a good personality and has been differentiated from Peach, so they aren't much alike anymore. Daisy has existed for almost two decades and still hasn't appeared in RPGs or recent Mario games. Princess Daisy could have a role in a mainstream game, and her kingdom, Sarasaland, hasn't been seen in so long that people don't even know it exists anymore. The fact that she hasn't been in another main game still pisses me off.

Toadette is a rather interesting little character. She was born in a spinoff but went on to appear in many RPGs, including the Mario and Luigi series and Paper Mario, and even made small cameos in some main games but still hasn't been in a true Super Mario title. Why? It confuses me because she already has her own personality and distinct traits.
She was proven to be main game material after her appearance in Captain Toad: Treasure Tracker, which shows she's very capable of appearing in an actual title where she has a good role.

Wario hasn't been in a mainstream game since the Super Mario 64 remake for the DS. He's had his own games, his own personality, and his own values. I don't see why he hasn't appeared in a modern Super Mario title yet, where he could be the antagonist, an ally, or an anti-hero. It would be nice to see him in another mainstream game with Mario.

Pauline hasn't shown up in Mario games since the Mario vs. Donkey Kong series, which went on until the last release on the Wii U. Pauline still exists and needs a proper return to the Mario games. She could get character development in mainstream games and appear in spinoffs. It's time for Pauline's comeback.

Birdo has been appearing in spinoffs and RPGs for quite a while, but when was the last time we saw her in a mainstream game? Super Mario RPG? Birdo is an interesting character who could have roles in main games, so she should definitely be in one.

Professor E. Gadd is a scientist who debuted in Luigi's Mansion and was an awesome character. While he doesn't appear in many games, his inventions have been seen in later games. He appeared outside the Luigi's Mansion series in the Mario and Luigi series. It's been years, so shouldn't he be in a mainstream game by now?

Where the hell has Yoshi been?! He's been absent from main games for a while now, so he needs to return.

We haven't seen much of this guy lately. Where did he go? He hasn't appeared in a main game since New Super Mario Bros. He was best known for his debut in Super Mario Sunshine.

Debuting in Luigi's Mansion and appearing in Super Mario 64 DS, King Boo has a lot of potential to become a main enemy in the Mario series.