Fridge Brilliance

The basement door locks from the outside and has a tendency to close on its own (and might even be spring-loaded). This seems like a dangerous flaw in the house design... until you remember that Frank was keeping people hostage down there, and likely intentionally had the door installed that way, or installed it himself, as a failsafe for any escapees.
It's a rather on-the-nose metaphor, but when discussing Tess' ex-boyfriend, Keith tells her "There's always gonna be people that project some kind of dynamic onto us that serves them." This is massive foreshadowing for pretty much every character aside from Tess - Keith had some romantic intentions towards Tess that she rebuffed, The Mother viewed her and AJ as her babies, Frank viewed the women and his progeny as a legacy, and AJ viewed himself as the good guy, and that it was just circumstances making him behave badly.
When Frank puts on his maintenance man uniform, the name tag clearly says "Carlos". Frank has a conversation with a neighbor and the neighbor does not seem to notice or care that Frank is wearing a uniform with the wrong name on it. This could be a subtle way of showing how Frank was able to disguise himself as an ordinary man with the rest of the world completely oblivious to the kind of person he truly was.

Fridge Horror

Andre mentions that the Mother is the result of several generations' worth of incest, with Frank raping his victim's daughters (which are also his own). However, we are shown Frank preparing for what seems like the first birth in the mid-80s. It's also fair to assume that, being seemingly fully grown and taller than most of the cast, the Mother is at least 20 years old. Because of this, it's likely that on top of being a rapist and incestuous, Frank is also a pedophile who raped his own (grand)daughters before they came of age.
The dress that AJ stumbles on in Frank’s underground room is the same one that worn by the woman in the flashback where Frank was casing a potential victim’s house.
Where are all of Frank's past victims or children in the present day? Frank likely killed the former, based on the bloody handprint in the rape room. The children, while Frank appeared to have some intention of keeping them alive, perhaps Mother kept trying and failing to care for them, or perhaps Frank killed them as well when they got too old or rebelled (either of which would help explain Mother's obsession). This may be a film about sexual violence and rape culture, but Frank is likely a Serial Killer to rival (pre-death) Freddy Krueger and John Wayne Gacy.