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).
Guts and Casca in some kind of fluffy romantic comedy setting. Those two deserve a rest.