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Thinking about YA girls friendship. For spring break, the girls take Luna on a tropical island getaway (and to take a break from the city). They take her shopping, surfing, zip lining, and more so she gets to experience having fun. They even come across Rachel Summers-Grey who is also on spring break.
Ahh yes! Also, Luna canonically loves the ocean after growing up on the Moon and she would enjoy that so much.
Honestly, though, my ultimate crossover would be all the legacy characters/children of older heroes coming together. A lot of them have so much in common and it would just be fun to see!