Nah, Luna as guest or either interactions with adult Noctis would ruin their tragic and dramatic relationship. Luna is like symbol that always out of reach for Noctis, childhood sweethearts is pretty standart trope for japanese people.
But if they show all the stuff she did before Altissia, awakening gods for Noctis, her relationship with brother, it would be awesome.
Ravus motivation is pretty clear, they wanted to show that he is a dick, but in the end all he cared about was her sister, and he just though that Noctis is not worth of her sacrifice that why he did what he did, that why he hate him. He even tried to get burden of a Chosen King, by putting ring on himself, but got denied by gods. I really wish they expand Luna and Ravus relationship, its really beatiful.
Also her relationship with Gentiana too, there is alot of stuff that they can do just by adding simple cutscenes