Tbh I feel like at some point all the NW girls will be added to the main series. Marvelous probably are waiting to do so with SK7 (which will likely be a new series)
The Gessen and Hebijo (Hebijo Clandestine, not the Crimson Squad who were the Hebijo girls in the 3ds series) came were known a bit before SK New Waves release, meaning the other girls could've been potential later additions too
Although for the Mikagura sisters, Kagura, Naraku, Jasmine, and Ryoki, the Estival Versus series was a special one that pretty much was during their summer vacation (hence why they were in a beach and why little to none of the stages were at their schools)
Now Ayame, Souji, Leo, and Yuyaki have officially been introduced. In addition to that
- Misato and Ushimaru's pets are used in PBS
- Kagari's outfit from one of her New Wave cards is
- Bashou constantly has her own merch thanks to her popularity
- Shinozuka girls cameo in SKBA
- ARC Angels seen in SK7
SK7 will likely be the beginning of a new series, so they can properly introduce these girls
Ikagura and Katsuragi delayed their graduation, so they'll stick to being 3rd year students alongside Asuka, with Yagyu and Hibari becoming 2nd year students. Miyabi and Imu were in a comma during the orginal series (Burst and Deep Crimson) so they'll continue as 3rd year students, alongside Murasaki, Ryobi, and Ryona as 2nd year students
That leaves room for the New Hanzo girls (Fuuma, Hijikata, Ayame, Seimei, Muramusa) as 1st year students in Hanzo, and the New Hebijo girls (Souji, Chitose, Ibuki, Ashiya, Bashou) as 1st year students in Hebijo
The Touno Tengu (Yuyaki, Misato, Kumi, Ushimaru, Kochi) and Zodiac Guiding (Leo, Ginrei, Kurohoro, Syuri, Aimu) girls will probably be the main girls for the new series.
As for the rest, they will probably help bring the "not everyone gets along" factor (since all the girls from Burst-Estival Versus have a bond now), so the true good and evil feel could come back. That could help with the SK7 theme which seems to be returning to the dark side
- Milky Pop girls are idols as a cover up for their real motives, their rivals could be the Arc Angels girls
- Maisen girls could show their experience with coming to Japan to become ninja, their rivals could be the Shinozuka girls
- Idk about the Sakigei and Metropolitian girls, but they could fit in too, especially the Metropolitian girls with their "elite" theme
- The Senki Shu girls would be the new main antagonists, so they'd have a big role. And they'd help emphasize on the evil factor more
With them possibly being added to SK7, SKPBS should still be alive. So maybe they'd add them after, during, or maybe even before SK7's release