Two characters of your choosing in some way have to cross over the other characters verse (or both verses merge/are revealed to be in the same reality) either temporarily or permanently. Both are completely in character with power and it affects the cannon of both franchises.
A Superman from one of the dc universes going to dragon ball and teaming up with Goku
I guess Yuki Terumi choosing Broly as his new host, both are green haired and crazy. Or Simon Belmont trying to hunt down Dio from JJBA. Another fun one would be Dante from Devil May Cry landing in the Fire Emblem Universe and trying to help out killing monsters
Nocturnal teams up with the The Lady of Pain (Dungeons and Dragons) to become more infinite to destroy the Aurbis (or Lady just BFR’s nocturne instantly, I doubt she would work would anyone even if the Dungeon Master forced her to).
Actually change the Lady of Pain bfr’ing to Nocturnal kills with a glare and that would be more accurate (and yes I know how to edit comments but I just want people to know what I originally commented)