Hellboy

Short Answer: Hellboy's kids; revisit the comics

There are only 3 movies in this series, and the most recent, the reboot, was considered... a disappointment. 

The thing to do would be to pretend it didn't happen and, instead, make the movie as part 3 of the first two-- the second one ends with Hellboy about to become a father. Of twins.

There are only a few ways to go with twins: both are good, both are evil, or one is good and the other evil. Of these options, the third seems the most interesting (and grounded in Biblical precedent). Upon hearing their father's stories, one decides the humans are mostly trustworthy, while the other follows the logic (brought up earlier in the series, but not fully explored): "Why are we fighting with humans against demons.... if we are demons?"

Further, he learns that Hellboy was designed to bring about the apocalypse, end humanity, and usher in the age of demons... but has decided against fulfilling that destiny.

Then the Evil Twin finds out that Hellboy was the son of the demon Azzael... and a human woman, Sarah Hughes. This is confusing to him, so he studies up on them. He learns that his his grandmother was taken to Hell and that his grandfather is imprisoned there. So he goes to Hell to find them and meet them personally. Either his grandmother was an evil human to have married a demon, or she was a good human who felt that even a demon was good enough to marry. It seems to him that either way, demons are better "people" than humans.

The Good Twin learns the same information but has the opposite response-- it's all shades of gray. Some humans are worse than demons, while some demons are not that bad. But right now, each has a realm, and that seems fair. He joins his twin on this trip to Hell to visit their grandparents. On the one hand, he wants to have an open mind. On the other, if Evil Twin is radicalized and wants to do something really bad, Good Twin wants to be there to stop him. 

Only once they are gone does Hellboy realize the boys have gone to Hell, so of course he has to go there after them. Ultimately, there is a family reunion, with all three generations in one place.

There is a series in the comics called Hellboy Goes to Hell, and that would make a pretty good title. In those comics, though, he does bring about The End of the World as a result... so that part might have to be rewritten. 

Our movie ends with Hellboy renouncing his destiny, and saying: there is no destiny-- only choice. But the Evil Twin remains unconvinced, and so we set up Part 4 with Evil Twin releasing Azzael from his prison in Hell.

But we have to move beyond Rasputin and the Nazis. Those plotlines are pretty much played out. Also, integrating the Hellboy story with Arthurian legend also seems... much. The urge to tie such characters into other mythologies is strong-- we have seen it play out in the Hercules and Xena shows, for instance. But Hellboy is part of Catholic demonology, and its own-- so let's focus on those for once. 

There are ways to integrate the plotlines of the graphic novels with those set up by the first two (acclaimed) movies, is the point.

It would also be cool to spin off Abe Sapien and connect him with the Creature from the Black Lagoon, and with the Amphibian Man from The Shape of Water, also. There could be a whole water-based series that is counterpoint to Hellboy's fire-based one...