Personally, I’d just love a one-shot about Asuka Langley Sōryū and her EVA-02 landing in the world of early Attack on Titan. That’d be epic.
A close second would be a short story about Saitama beating up the entire Dragon Ball universe because of a misunderstanding (kinda like the concept behind the Granolah arc).
Tai with Agumon in Pokemon - childhood fantasy never going to be realized xD
Otherwise Momonga (Overlord) in any other Isekai as final boss/enemy (i.e. A wild last boss appeared! - Yasei no Last Boss ga Arawareta!)