An Alternative Literary History

Here is another pseudo- or proto-thought about genre interfaces. Imagine that:

sf and maybe other genres did not get ghettoized into magazines for decades;

writers like HG Wells, ERB, Kipling and their successors continued to write general fiction including occasional works that we would recognize as science fiction.

Would the line, if any, between genres be drawn in a different place? Might some otherwise mainstream novels have included background details that we would call sf but that did not impact directly on the plot? Poul Anderson's detective novels contain passages demonstrating that the author thinks like, as he indeed is, an sf writer. See here, here and here.

I would like to see:

in a mainstream novel, Manson Everard in civilian life;

in a historical novel, a trader or other worthy citizen who, we knew from other works, was a disguised time traveler;

a novel about scientists in which some of the supporting characters were making discoveries beyond the current level of scientific knowledge;

a mainstream series in which the characters discovered a buried ancient spaceship under their street in the final installment.

After all, if such a discovery is to be made, then it will be made by people whose lives have until then been what passes for normal. I think that I dreamed that last example a long time ago. It would be the author's job to make the conclusion seem plausible, not arbitrary or deus ex machina. But let's push the boundaries a bit.