When are you going to have kids, Henry Frankenstein? Your dad wants to know.
That's the end of Frankenstein. The old pompous Baron cheerfully contemplates the marriage of Henry and Elizabeth and tells the servants,
"As I said before, I say again, here's to a son to the House of Frankenstein."
At first it seems like an odd place to end the film; who cares about sons? The whole film's about monsters and horror and brains. Where does this sudden interest in heirs come from?
But think about it for a second. Henry already created a son in the House of Frankenstein. He made his own child out of stitched together parts of dead people. Which isn't the usual way you're supposed to do it, obviously.
In hoping for a regular son produced the old-fashioned way, via babymaking, the Baron is saying, "Hey, everything's all right now. Nature is reasserting itself, and my son, Henry, is going to be a regular guy in a regular way, marrying and making children, rather than trying to run away from his manly duties by fiddling with electricity and trying to make a son all on his own out of dead bits."
The Baron's concluding hope for a son is also kind of ominous and weird, though. It emphasizes, again, that there already was a son in the House of Frankenstein. Henry's drive to create life for his own glory went very badly. Will the Baron's eagerness to create life for his glory go better?
You could see the end of Frankenstein as a kind of nudge-nudge warning. The cheerfully empty-headed nobility will continue to produce sons, and those sons will continue to terrorize the countryside, blandly destroying your lives on a whim. The rich and bored will unleash their hobbies on you, and make your life miserable.
Watch out, peasants, the Baron is saying. Frankenstein is going to make more monsters.