Ok so Phantom Girl.
She's from the Legion in the 30th century in DC comics right? Her main power is intangibility (with smth I read saying that she later gained the power to shoot dark matter or smth??? Pretty cool but not the focus here) and her ability is so strong that in the original comics of her she used it to come to earth by phasing through the phantom zone and dimensions.
So
It would be easy for her to accidentally phase through reality and into the Ghost Zone right?
Just imagine all the ghosts being slightly freaked cuz this girl has the powers of a ghost but she lives. Not knowing what to do with her they direct her to the only other living person (sorta) that has the same abilities.
Now Danny has to figure out either
A) how to get back to her own time cuz she also kinda time travelled by phasing between realms
Or
B) where the closest natural portal would be to get her home without discovering this ghost city forgotten by the advancements of the world.
Just a funny lil thought.