The whole film is Maria's hallucination
Being the sole person responsible for leading La Résistance and keeping it peaceful has to put pressure on Maria, and so she spends months or years waiting for "the mediator" she hopes will take the burden of responsibility from her. However, just before the "events" of the film take place, something brings her to the devastating realization that the mediator will never materialize.This realization causes Maria to have a complete mental breakdown. Rather than come to grips with reality, she projects her hopes onto a random boy she saw one day - Freder - and lapses into a dream world where he is the central character. This explains the overall fantastic and improbable nature of the events that take place. The heavy symbolism present throughout the film is a sign of the way she sees the world in mythical/Biblical terms, like the parables she tells. It also explains Maria's character decay from independent rebel leader to pathetic damsel in distress; she's overwhelmed by the stress of leading alone and wants to surrender that power to someone else. The robot subplot is an expression of her fear of her own sexuality. The simplistic way everything is resolved reflects her naive view of the world in general and labor disputes in particular.
- This theory is somewhat undermined by the longer restored cut, which adds some complexity to the ending, and also showcases Maria repeatedly risking her life to save others, especially children, even late in the movie.
The Heart Machine is a prison for an Eldritch Abomination.
In ancient times, humans were sacrificed to keep the creature sated, remaining dormant in its lake of blood. In modern times, the temple was rebuilt into a vast machine. When a portion of it exploded, Freder was granted a vision of this era. Now, the Heart Machine pumps blood collected from workers, hospitals, and elsewhere, redirecting it all to the dark god inside it.- The Heart Machine is not the Moloch Machine, though that is not made clear until the rebellion at the end. However, the two are probably connected in some way since the Heart Machine is implied to be central to all of the city's machinery.
- It's red mercury and not blood. The machine is one of the first on-screen concept of an atomic powerplant.
"Der Schmale" (the thin one)...
...is related to The Slender Man Mythos.- Now that you mention it, he does spend most of his time stalking the protagonist and his friends, albeit on Fredersen's orders.
- Perhaps this is why all but one of the scenes in which he appears are conveniently missing from any surviving cut of the film...
The film is an Interquel to The Time Machine
The film takes place in 2026, 131 years after the Victorian era present of the novel (1895) and 800,675 years before the time to which the Time Traveller travels (802,701). The wealthy elites live aboveground and while away much of their time in idleness and childish play, while the workers toil endlessly underground. These are the exactly the general social trends that the Time Traveller theorized led to the former becoming the Eloi and the latter becoming the Morlocks. Really, the only thing you have to do to make the two fit seamlessly is to assume that the happy ending to Metropolis with "The Heart" as the mediator to the Head and the Hands doesn't end up panning out in the long run.- Alternatively — if you want to keep Thea Von Harbou's message intact — the actions of Freder and Maria created an alternate timeline where the Bad Future in the Time Machine was avoided.
Rotwang is Freder's Real Father
Rotwang's primary motivation for the majority of the film is that he hates Joh Fredersen because his love interest, Hel, left him for Joh and then died giving birth to Freder. The exposition, especially in the novel, Hel and Rotwang's breakup and Hel's death occurred in fairly rapid succession. Unless human gestation periods change significantly between now and the year 2419, this would seem to imply that Freder must have been conceived *before* Hel left Rotwang for Joh. While it's possible that she may have been sleeping with Joh in secret while still ostensibly dating Rotwang, this seems unlikely based on what we know of the characters.- Oddly fitting, considering, that the Start of Darkness character-arc for Darth Vader (the definitive example of the relevant trope) as provided by Revenge of the Sith seems to be lifted directly from Rotwang's backstory in Metropolis (Joh's internal monologue even states at one point that though Hel technically died *in* childbirth, she died not from complications but from a broken heart after Rotwang drove her away, a plot device that probably seemed a lot less corny in 1927).
- While this theory sounds quite plausible, (especially since Rotwang is pretty much Freder's Evil Counterpart, while Joh Fredersen is his complete opposite,) there's still the fact that In-Universe, everyone considers it a given that Freder is Joh's son. Thus, the timeline pretty much has to add up... somehow.
- This theory puts a very, very dark twist on Rotwang being so very ready to not just mentally wreck but even kill Freder.
There are two Heart Machines, the one that got destroyed powers Metropolis and the second one powers the Worker city
When Fredersen calls Grot's station, it's clearly labelled as "HM 2" - "HM" most likely standing for Hertzmaschine / Heart Machine. Even after the Heart Machine breaks down and Metropolis enters a blackout, we can see that the lights in the Worker city are still working.Heart Machine 2 also powers the pumps which prevent the Worker city from flooding. This is likely an intended feature meant as a safeguard - if the workers revolted and cut Metropolis off power, their city would get flooded.