Dorayaki
King Bowser
It's similar with another topic "old characters that you would like to see in a new game", but talking about the Mario chracter history.
Nowadays there are many complaints about bad rosters in Mario spinoff games, because of the improper use of main characters.
But, if the staff had a better foresight of making spinoffs in 1990s, would there be not only better rosters, but different main characters?
Excluding main characters who already appear before SMB (Mario bros, Peach, Toad, DK, Bowser), take a look at how each main character made their spinoff debut, classified by games:
Playable debut in Super Mario Kart, 1992: Yoshi
Yoshi: the first ever added main character, after his real debut in SMW, 1990, Yoshi was added to be the earlies Kart game.
Playable debut in Kart 64, 1996: Wario
Wario: after his real debut in SML2, 1992, actually he was already playable in his subseries like Wario Land, Wario Woods. Though the previous Super Mario Kart was after SML2, Wario wasn't immediately added to this game, like Daisy in SML.
Playable debut in Golf 64, 1999: Baby Mario
Baby Mario: surprisingly earlier than many other main characters. His debut was in the origin of Yoshi series, SMW2, 1995. This also make other new babies playable in spinoffs.
Playable debut in Tennis 64, 2000: Birdo, Daisy, Waluigi
Birdo: her origin was in SMB2(USA version), 1988, which is earlier than her counterpart Yoshi. But obviously, it's very late for her to be playable as Yoshi already had several own games in that time.
Daisy: her origin was in SML, the 1989 gameboy game, where she was a replacement of Peach. Even though it's earlier than Wario, her playable debut is later than him. Her official portrait also appeared in NES Open Tournament Golf, 1991. The 1993 film Mario bros was possibly another factor to let Daisy become more important in Mario series.
Waluigi: the first ever spinoff original main character (Uh). Generally he seems to be created as Wario's partner in face of Mario bros.
Playable debut in Mario Golf Toadstool Tour, 2003: Bowser Jr.
Bowser Jr: created in SMS, 2002. He's also the character that only needs one year to become playable.
Playable debut in Mario Kart DD, 2003: Toadette, Diddy Kong
Toadette: the second spinoff original main character. But it's weird for her status to be an original spinoff characer.
Diddy Kong: a subseries character, debut in DKC1, 1994, and had his own spinoff: Diddy Racing in 1997.
Playable debut in Mario Superstar Baseball, 2005: Dixie Kong
Dixie Kong: the thrid DK character, debut in DKC2, 1995.
After these information, we can see some spinoffs were the key to determine tha future Mario main characters, from the 1990s. What about the other characters from the same timeline?
Pauline: the very early character, and still recurring, but still no playable chance. Her major problem was to be regarded as a prototype of Peach in NES era, but she was designed to be a different character in 2000. Her example was actually similar with Daisy's, but the staff could just say that she's another princess and Peach's friend.
Foreman Spike: a character appeared in Wrecking Crew, 1985, and playable in Mobile Golf, 2001. He shares some similarity with Wario, but no knowing why the staff changes the idea with Wario.
The Super Mario Bros. Super Show characters: appeared in 1989. In this story there are some puppets characters and many related characters referred to be Mario and Luigi's friends/relatives, though the staff chose none of them to be reused in spinoffs, official introduction or Yoshi series. Some cousin/nephew characters are actually possible to replace baby characters (if they were young enough).
Koopalings: appeared in SMB3, 1993. Actually they had a good time in comic and cartoon era as official main characters. Their major problem was to be a group, as the staff didn't choose them to be individual spinoff character.
Wart, Tatanga: the two bosses from the early SMB2(USA ver) and SML face the major problem of being with Bowser, where the staff seem to think only one major boss can be in spinoff series. I think too many bosses do kill diversity, however, Fawful is an intersting example.
Geno, Mallow: the very first main RPG characters in Seven Star, 1996. They seem not to be reused because of the copyright problem, but other future RPG characters aren't reused too.
Gene: actually Gene was a recurring player in golf games even earlier than Daisy or Birdo. It's unknown why the staff cancelled his chance in more sport games.
Human athletes: Tennis 64 was also a notable point of Mario history. The staff (Camelot) stuffed many human athlete characters along with Baby Mario in Golf 64, but suddenly in Tennis 64 they changed their mind and put other old Mario characters/species and make them recrurring. It's unkown why the two similar games have such two opposing rosters. If the athletes still appeared in Tennis 64, we can see them in Olympics now.
Compared to these unused characters, the added main characters (Wario bros, Yoshi, Daisy) have somewhat a common point: they have similiar styles with the original main characters. However, it also leads to less diversity (many plumbers, toads, alteregos etc). Though we can't change the history, the only way the staff should do is using stories to prove that the added main characters are more competent than unused ones.
Nowadays there are many complaints about bad rosters in Mario spinoff games, because of the improper use of main characters.
But, if the staff had a better foresight of making spinoffs in 1990s, would there be not only better rosters, but different main characters?
Excluding main characters who already appear before SMB (Mario bros, Peach, Toad, DK, Bowser), take a look at how each main character made their spinoff debut, classified by games:
Playable debut in Super Mario Kart, 1992: Yoshi
Yoshi: the first ever added main character, after his real debut in SMW, 1990, Yoshi was added to be the earlies Kart game.
Playable debut in Kart 64, 1996: Wario
Wario: after his real debut in SML2, 1992, actually he was already playable in his subseries like Wario Land, Wario Woods. Though the previous Super Mario Kart was after SML2, Wario wasn't immediately added to this game, like Daisy in SML.
Playable debut in Golf 64, 1999: Baby Mario
Baby Mario: surprisingly earlier than many other main characters. His debut was in the origin of Yoshi series, SMW2, 1995. This also make other new babies playable in spinoffs.
Playable debut in Tennis 64, 2000: Birdo, Daisy, Waluigi
Birdo: her origin was in SMB2(USA version), 1988, which is earlier than her counterpart Yoshi. But obviously, it's very late for her to be playable as Yoshi already had several own games in that time.
Daisy: her origin was in SML, the 1989 gameboy game, where she was a replacement of Peach. Even though it's earlier than Wario, her playable debut is later than him. Her official portrait also appeared in NES Open Tournament Golf, 1991. The 1993 film Mario bros was possibly another factor to let Daisy become more important in Mario series.
Waluigi: the first ever spinoff original main character (Uh). Generally he seems to be created as Wario's partner in face of Mario bros.
Playable debut in Mario Golf Toadstool Tour, 2003: Bowser Jr.
Bowser Jr: created in SMS, 2002. He's also the character that only needs one year to become playable.
Playable debut in Mario Kart DD, 2003: Toadette, Diddy Kong
Toadette: the second spinoff original main character. But it's weird for her status to be an original spinoff characer.
Diddy Kong: a subseries character, debut in DKC1, 1994, and had his own spinoff: Diddy Racing in 1997.
Playable debut in Mario Superstar Baseball, 2005: Dixie Kong
Dixie Kong: the thrid DK character, debut in DKC2, 1995.
After these information, we can see some spinoffs were the key to determine tha future Mario main characters, from the 1990s. What about the other characters from the same timeline?
Pauline: the very early character, and still recurring, but still no playable chance. Her major problem was to be regarded as a prototype of Peach in NES era, but she was designed to be a different character in 2000. Her example was actually similar with Daisy's, but the staff could just say that she's another princess and Peach's friend.
Foreman Spike: a character appeared in Wrecking Crew, 1985, and playable in Mobile Golf, 2001. He shares some similarity with Wario, but no knowing why the staff changes the idea with Wario.
The Super Mario Bros. Super Show characters: appeared in 1989. In this story there are some puppets characters and many related characters referred to be Mario and Luigi's friends/relatives, though the staff chose none of them to be reused in spinoffs, official introduction or Yoshi series. Some cousin/nephew characters are actually possible to replace baby characters (if they were young enough).
Koopalings: appeared in SMB3, 1993. Actually they had a good time in comic and cartoon era as official main characters. Their major problem was to be a group, as the staff didn't choose them to be individual spinoff character.
Wart, Tatanga: the two bosses from the early SMB2(USA ver) and SML face the major problem of being with Bowser, where the staff seem to think only one major boss can be in spinoff series. I think too many bosses do kill diversity, however, Fawful is an intersting example.
Geno, Mallow: the very first main RPG characters in Seven Star, 1996. They seem not to be reused because of the copyright problem, but other future RPG characters aren't reused too.
Gene: actually Gene was a recurring player in golf games even earlier than Daisy or Birdo. It's unknown why the staff cancelled his chance in more sport games.
Human athletes: Tennis 64 was also a notable point of Mario history. The staff (Camelot) stuffed many human athlete characters along with Baby Mario in Golf 64, but suddenly in Tennis 64 they changed their mind and put other old Mario characters/species and make them recrurring. It's unkown why the two similar games have such two opposing rosters. If the athletes still appeared in Tennis 64, we can see them in Olympics now.
Compared to these unused characters, the added main characters (Wario bros, Yoshi, Daisy) have somewhat a common point: they have similiar styles with the original main characters. However, it also leads to less diversity (many plumbers, toads, alteregos etc). Though we can't change the history, the only way the staff should do is using stories to prove that the added main characters are more competent than unused ones.