The rotoscoping is still far from perfect, but given that I had to rotoscope dark characters from a dark background, I think it turned out surprisingly well.
I saw someone suggested making the background dark like moonlight. I was thinking the portal you could use could also resemble the portals in the World Between Worlds from Rebels. I loved that concept and it would be an easy way to use a darker transition.
https://youtu.be/v254mgxMmG8