Crossovers

Can there be plausible crossovers between fictional series of different genres? Not often. Nicholas van Rijn can visit the Old Phoenix but an appearance by a fantasy hero would be an unwelcome intrusion in a hard sf series. We can accept that the various universes coexist in a single multiverse but usually do not interact. If there were to be interactions, then they could be allusive and ambiguous rather than in your face, e.g., one and the same character might be a merchant in a historical novel and a time traveler posing as a merchant in a time travel novel.

The superhero genre is ahead of the game in this kind of interaction:

heroes with scientifically based powers meet heroes with magically based powers;

when Plastic Man, drawn in cartoon style, met Superman, drawn more realistically, it was explained that the drug that caused Plas' stretching power also distorted his perceptions so that Supes looked cartoonish to him;

when Alan Moore was editorially instructed that his fantasy series, Swamp Thing, had to participate in the company-wide crossover of the Crisis on Infinite Earths, he explored the supernatural after-effects of the Crisis.

Lessons might be learned by writers of prose fantasy and sf. A Crisis in Poul Anderson's Old Phoenix multiverse might manifest in different ways in the diverse universes without compromising the integrity of those universes. Trygve Yamamura solved cases that seemed to have a supernatural element. Maybe that element was present on a level that he did not detect but that was known to Valeria Matuchek.