A spinoff focused on the other person summoned in the summoning ritual, Aira. Honestly, while I love the idea of a story centered around Aira (she's just a poor high schooler who was summoned to another world and she's not even the saint; she left her family and friends behind, it's hard not to feel bad for her), I feel like this side story is very much lacking. For one thing, much of what's in this manga is already covered in the main series! With no new thoughts/devlopments/characters. We could have seen more of Aira's trauma of being summoned to another world, because unlike Sei, she actually still had attachments to Japan. We could have seen flashbacks of her old life on Earth. But we really don't get that.
Things sort of pick up after the confrontation between Kyle and Elizabeth, and Kyle gets grounded by his father. We see that Aira is now under the protection of the second prince, Rain, and we see Aira's friendship with Elizabeth (while Sei is great and all, it's nice for both Aira and Elizabeth to be friends, because they are both teenagers while Sei is a ~20 something office lady). Aira endures herself to the other female students at the magic academy, and her reputation improves from the rumors/gossip that were spread about her. Y'see, that's the kind of thing I like to see. I liked seeing Aira asking for help from the other female students, and the other students complimenting her about how she's so smart/a top student from the academy. I hope the author creates more original content for Aira in this manga. For example, new characters aka other students that Aira befriended at the academy.
The main series, which is ahead of this spinoff, shows us that Aira joins the Magi Assembly after graduating the academy. She's not the saint, but she is a very talented mage, with multiple attributes (it's rare for mages to have more than one). We can learn things about the world from Aira's perspective and not Sei. While Sei is being taught by the Grand Mage Yuri, it's not like she's a member of the Assembly like Aira. She's staying at the Research Institute, since she likes herbs/alchemy. Through Aira, we can learn more about the Magi Assembly, meet the other mages at the Assembly/new characters, etc.
A spinoff manga can be done well. Just look at My Next Life As A Villainess: Verge of Doom. In that series, instead of the MC Katarina recalling her memories as a child, she does when she's a teen and already at the setting for the game she's been reincarnated in. Thus, she has less time to avoid her doom as a "villainess". In that spinoff, we meet new characters like Sienna, a follower of Katarina, and friend of the new Katarina. It's my hope that the author for "The Saint's Magic Power is Omnipotent" can write the spinoff for Aira better and utlize more interesting elements.