So, I don't really know where I'm going with this, not that that hasn't occurred before. I'll just wing it, and if it ends up being really simple or really complex, then it is what it is. I've noticed that, in general, not many crossovers with Genshin Impact exist, much less ones with the new Sumeru characters. So I'm hoping that, with this, the likelihood of such a crossover occurring will be at least a bit higher, though I haven't actually seen anyone adopt any of my ideas, much less the more unusual ones…
Regardless of that, imagine characters from Sumeru like Alhaitham, Cyno, Dottore, Candace, Dehya, or Nahida being transported into the world of Highschool DxD. Now, imagine if they were all working together as one group, and that is this idea. (Yes, even Dottore.) I might create another idea later with them spread out in the world of Highschool DxD rather than them all being together, but you'll just have to wait and see, I suppose.
I think Nahida would be the leader of the group, as she is the strongest, being a god and all, not to mention her being the god of wisdom. After that, I'd say that Cyno should be the second in command of the group, that Alhaitham should be the go-to-diplomat for situations, and that Dottore should be the group's scientist, although his personality may have to be toned down or altered in order to fit in with the rest of the group. I'm not sure about the other characters' positions yet, and you're free to rearrange anything you dislike, such as what positions I've chosen for the characters, or if you think there's someone better suited to those positions.
I don't know if the group should stay as humans or not, nor do I know what the name of the group should be, so I'll leave that up to you. If you make the group a devil peerage, then these are the evil pieces I'd recommend for each of the characters. The king for Nahida, the queen for Cyno, a knight for Alhaitham, a bishop for Dottore, a rook for Dehya, and the other knight for Candace. As for the other positions in the peerage, if you want you can either have the characters require multiple evil pieces to reincarnate them as devils, or you can just have not be used. If you want to add non-Sumeru characters later on, that is fine as well. Maybe have Zhongli as the other rook, and Albedo for the other bishop. As for the pawns, you can put whoever you'd like from there. Maybe even have some Highschool DxD characters fill in those positions. You can rearrange any of this, if you believe that my placement of characters needs some fine tuning or adjustments.
Lastly, just have the characters interact with the world of Highschool DxD. Don't just limit the group to interacting with one other group, and occasionally others when necessary for the plot. Or, if you do want to do that, then at least don't have it be Rias's group that they have that interaction with. Make the other main people be people like Yasaka, Amaterasu, Azazel, or someone else. Though I do understand it's necessary for the plot that Rias and her group to be involved at key moments of the story, they don't always need to be a part of things.