The team I would personally enjoy the most to see:

She didn't get a skin for a while, actually expected her to finally get that highly requested Project skin, but well... She works for Star Guardian, too, in my opinion. Already has a familiar with Valor and we could use some more tomboyish Star Guardians (hoped for Vi to be that first, but Caitlyn got Arcade and Vi should get the Star Guardian skin the same time like Cait, they should be in the same team).
Could be one of the most interesting Star Guardians, because of a special background. I imagine her as a veteran who got heavily injured on a mission, close to death, but the first star managed to save her by granting her a new, magical, artificial body. Also already has something that can serve as her familiar with her ball (armadillo familiar?). Also brings a new personality trope to the team: kuudere (appearing emotionless and cold at first, but is loving in a subtle way).
Highly requested for a long time by her mains and she would fit the skin line well, that's all.
Didn't get any non-esports skin until now and could definitely use one. Her personality goes well with the Star Guardian theme, but they could also make her a bit different, like a himedere (acting like a heighty princess), since this would go well with a kimono based design (which would fit her sleeves well). Would also make her more standing out in terms of her magical girl outfit.
Is basically already confirmed to get the skin and a prestige edition. Also, since she clearly has a thing for cute girls, she could be quite fun to read about in the Star Guardian lore, always trying to make a move on the other girls.
For the legendary, because of personal bias, I would like Orianna to get it, especially since her original voice over is so .... weird. Additionally, it could flesh out her tragic past better if she would get new voice lines.
Close runner up would be Taliyah, for the sole purpose of changing her personality into that himedere I was talking about. But I prefer the robot girl....