Mentioned this in the other thread. If it's a fight based on a collaboration, then it will most likely be a PSO2 trial.
I would love to see a Profound Darkness or the corrupted version of Matoi (forget her actual name, Falz Angur maybe???)

PSO2 recently got Odin.
https://www.youtube.com/watch?v=QPD7k2GvFYs

(Fight looks completely baddarse by the way. Much better than our version if I am honest... for those of you that played PSO2).

Will probably be Nidhogg though.. Or Evil red guy *insertcertainNPCnameheretoavoidspoilers*.