My theory is that it's 100% certain that Lucy and Rebecca are coming back in the future after the collab ends

They are in the crossing stars And they have lots of investments 2 r5s 2 s6s

My main comparison that I'm drawing is aloy from genshin she was a crossover in 2021 where you got a character & weapon that has no sequences/constallations or refinements.

The main difference is Lucy shows on the crossing stars if you don't have her

Where if you don't have aloy she does not show in the character archive/crossing stars