It occurred to me when seeing this cover.....
In my early memories of spin-offs or game covers, Princess Peach often stands for the "only" heroine of Mario members, until the era of GC when several female characters became clearly recurring. It may be owing to the essence of Mario main games which correspond to the tranditional RPG, that a princess is saved by a brave hero (male) from the evil boss (male). Nowadays, new mainstream of RPG don't use this model anymore, though Mario stands for the classics.
Still, Mario series didn't fall down for this typical RPG model and disgusted by female players. One is that Mario series hold the main idea of easy plot setting and playable details of games, which is easy for female players to play and adopt without consciousness of gender.
Another is that the Mario characters, species or background share a simple, funny or even cute designs in favor of girls' taste, in other words they are less violent that other action games. In this point, other popular Nintendo series such as Kirby, Pokemon or Zelda Legends (toon type?) are also adoptable among female players, because they hold cute and adventurous properties at the same times.
Back to the topic, the only pity may be the overweight status of the only heroine for Princess Peach, which is led by the RPG model. But is it realy unsolvable? If not violating the idea of main plot, the components of Mario members......
recurring species : except for characters, most species such as villians have individual sex but not mentioned by each one, so they don't need attention if we suppose they account for both two genders.
plumbers : must not be to please housewives (or househusbands?) so many male plumbers appear. It seems to be unfair, but has some rules. Begin with the first hero Mario, Luigi is his twin brother from the idea of 2P, Wario is his rival since GB era (if rivals in the same gender didn't relate to equality) , and Waluigi who corresponds to the previous two was created. For diversity of characters, there are also actually no need for another female plumber. I'm a little curious about what if Nintendo gave Mario and Luigi another little plumber sister since the old Mario TV show?
royalty : we can't discuss it since those princesses are the most recurring heroines and can't fade, though few human princes appear in Mario series, and it's also fair for princesses as heirs to throne. Thanks to Daisy's recurring, Princess Peach gets her first female human friend though in the same identity.....
toads : as an additional role from RPG model, the gender of Princess Peach's loyal minion seems nonspecific. If not a boy Toad appeared since SMB or SMB USA, oppositely the maid Toadette could appear earlier that Toad. Also since Toadette's debut in MKDD, her status is still not ensured as significant as Toad in spin-offs?
bosses : the most recurring boss, Bowser is male, if not violating the RPG model. Or in the sense of love triangle (if exists), Bowser and Donkey Kong must be male in favor of Princess Peach and Pauline. For Bowser Jr., who is Bowser's child, is aimed to be like him so reasonably being male. But for minions, only one girl consisted in Koopalings is not natural. Except for Bowser, there are also some featuring female rivals in Mario series single games, such as Captain Syrup or Shadow Queen. Maybe it's considerable to make another female boss recurring.
dinasours : uh, in this story they are transexual. If judging stereotype or mental sex, suppose that Yoshi is male and Birdo is female. Yet though Birdo appeared earlier than Yoshi, it's wierd that Yoshi is more major than Birdo in recent games, either main games or spinoffs?
kongs : it's also not told why Donkey Kong's little partner is a boy, who is recurring with Donkey Kong as guests in Mario spinoffs and Smash bros. If Dixie Kong hold the first status in DDC, Diddy Kong could still reversely appear since DDC2. Another problem is that Candy Kong becomes a fading heroine......but generally, in DK series itself, the problem is less serious than Mario.
subseries : main Mario members seem to be unbalanced, but surprisingly other original subseries skip this problem. For Sport series, volunteers of athletes-to-be are boys and girls by half. For Warioware, not only female mambers join, but with various identities. For Paper Mario, villages of species aren balanced for sex, and even same with Mario's partners. Maybe some featuring female characters from subseries can join in the main series to solve this problem.
As stated above, the gender consciousness problem in Mario is not serious in face of many other action games of Wii and Xbox in fact, and Mario games have other properties to attract female players as well. But as the topic fading heroines referred, actually present main heroines or other female characters from subseries is not insufficient, it's just the problem that Nintendo haven't make use of their designs. Also if this problem can be improved, not only more choices for female playrs, but possibly a pioneer to encourage other famous companies to follow.
it's impossible or meaningless to force the gender ratio really be 1:1 as there are originally fewer female characters
but basically as the main and DK characters defined, according to fair roster, actually only Peach and Daisy is recurring more than 50% chances
that means the rest main cast: Birdo, Toadette, Rosalina ,Dixie, Candy and minorly Pauline, Tiny aren't recurring at all
before the need of new characters or using subseries, these girls are target to be solved