Cinemonster's original list
*****************************************************************
six (6) countries
1. Shivers (1975) [Canada]
2. Inferno (1980) [Italy]
3. Funny Games (1997) [Austria]
4. 28 Days Later (2002) [UK]
5. [REC] (2007) [Spain]
6. I Saw the Devil (2010) [South Korea]
*****************************************************************
six (6) decades
1. Shivers (1975) [1970s]
2. Inferno (1980) [1980s]
3. Funny Games (1997) [1990s]
4. 28 Days Later (2002) [2000s]
5. I Saw the Devil (2010) [2010s]
6. The Bride of Frankenstein (1935) [1930s] [also one of my wife's favorites]
*****************************************************************
seven (7) films that are the second installment of their respective franchises
1. The Bride of Frankenstein (1935)
2. Damien: Omen II (1978) [also one of my wife's favorites]
3. Halloween II (1981)
4. Aliens (1986)
5. Hellbound: Hellraiser II (1988)
6. Child's Play 2 (1990)
7. Scream 2 (1997)
*****************************************************************
four (4) body horror films
1. Eraserhead (1977)
2. The Thing (1982)
3. Videodrome (1983)
4. Antiviral (2012)
*****************************************************************
two (2) films from this year
1. Host (2020)
2. Relic (2020)
*****************************************************************
three (3) "disease based" films
1. Shivers (1975)
2. 28 Days Later (2002)
3. [REC] (2007)
*****************************************************************
the highest rated film from the 1950s, according to Letterboxd’s users, that we can access
1. The Night of the Hunter (1955)
*****************************************************************
one (1) film that is set entirely inside one location
1. Funny Games (1997)
*****************************************************************
one (1) "Invisible Person" film
1. The Invisible Man (1933)
*****************************************************************
one (1) Hammer Film Productions film (no Dracula)
1. The Devil Rides Out (1968)
*****************************************************************
two (2) films by a black director or featuring a predominantly black cast or lead
1. Candyman (1992) [black lead]
2. Blade (1998) [black lead]
*****************************************************************
one (1) film with a movie theater in it
1. The Blob (1988)
*****************************************************************
and one (1) Tobe Hooper film
1. The Funhouse (1981)
*****************************************************************
and since we ended up with only 26 movies after satisfying all of the above criteria, we added the following films (all of which also meet or otherwise relate to at least one of the criteria above) to reach the magic number of 31 (actually 38, in our case, but who's counting?!) films:

1. Phantasm II (1988) [second installment of franchise]
2. Final Destination 2 (2003) [second installment of franchise]
3. Possessor (2020) [body horror] [this year]
4. Becky (2020) [this year]
5. Spree (2020) [this year]
6. The Beach House (2019) [disease]
7. Cube (1997) [one location]
8. 1408 (2007) [one location] [also one of my wife's favorites]
9. Let Me In (2010) [Hammer Film Productions]
10. Green Room (2015) [one location] [also one of my wife's favorites]
11. Tales from the Hood 2 (2018) [black director and predominantly black cast]
12. Scary Movie (2000) [movie theater]
*****************************************************************
extra credit per Cinemonster
1. The Witch Who Came from the Sea (1976)
2. Cube 2: Hypercube (2002)
*****************************************************************
and additional extra credit just because
1. Rosemary's Baby (1968) [the one film we set out to watch during Hooptober 5.0 (2018) that we failed to see]
2. Final Destination 3 (2006) [the one film we set out to watch during Hooptober 6.0 (2019) that we failed to see]
3. Suspiria (1977) [rewatch prior to watching sequel for Hooptober requirement]
4. Halloween (1978) [rewatch prior to watching sequel for Hooptober requirement]
5. Alien (1979) [rewatch prior to watching sequel for Hooptober requirement]
6. Scream (1996) [rewatch prior to watching sequel for Hooptober requirement]
7. The Little Shop of Horrors (1960) [2020 anniversary film]
8. Psycho (1960) [2020 anniversary film]
9. Cannibal Holocaust (1980) [2020 anniversary film]
10. Arachnophobia (1990) [2020 anniversary film]
11. American Psycho (2000) [2020 anniversary film]
12. Final Destination (2000) [2020 anniversary film]
13. Ginger Snaps (2000) [2020 anniversary film] [also one of my wife's favorites]
14. Black Swan (2010) [2020 anniversary film] [also one of my wife's favorites]
15. I Spit on Your Grave (2010) [2020 anniversary film]
16. Tucker and Dale vs. Evil (2010) [2020 anniversary film
17. The Lighthouse (2019) [best horror of 2019]
18. Midsommar (2019) [best horror of 2019]
19. Us (2019) [best horror of 2019]