My first year doing this. Cinemonster's rules are here, and this is my list of films I intend to watch (and review) over the course of October (culminating in a Rob Zombie's Halloween double-bill on the 31st), along with their respective categories; some films satisfy multiple criteria (and some are rewatches).

6 Countries:

[REC] - Spain.
Killer Sofa - New Zealand.✓
The Vanishing - Netherlands.
Cronos - Mexico.✓
Slaughterhouse Rulez - UK.✓
House - Japan.✓

6 Decades (not sure about this category, it sort of takes care of itself? If I make it through the list with days to spare, I'll add some separate films for this category):

The Invisible Man - 1930s.✓
The Bat - 1950s.✓
Dr Phibes Rises Again - 1970s.✓
The Vanishing - 1980s.
Cronos - 1990s.✓
Saw - 2000s.✓

7 Second Films of Franchises:

Phantasm II.✓
Halloween II (2009).✓
Dr Phibes Rises Again.✓
Creepshow 2.✓
Hello Mary Lou: Prom Night II.✓
28 Weeks Later.
The Dead 2.✓

4 Body Horror Films:

Brain Damage.✓
Antiviral.✓
Cronos.✓
Audition.✓

2 Films From This Year:

I'm Thinking of Ending Things.✓
Gretel & Hansel.✓

3 Disease-Based Films:

[REC].
28 Days Later.✓
28 Weeks Later.

The Highest-Rated Horror Film From the '50s That You Haven't Seen and Can Access:

The Bat.✓

1 Film That is Set Entirely Inside One Location:

Saw.✓

1 Invisible Person Film.

The Invisible Man (1933).✓

1 Non Dracula Hammer Film:

Captain Kronos: Vampire Hunter.✓

2 Films With a Black Director or Predominantly Black Cast or Lead:

Night of the Living Dead (1968).
The First Purge.✓

1 Film With a Movie Theater In It:

Interview With the Vampire.✓

And 1 Tobe Hooper Film (there must ALWAYS be a Hooper film):

Haven't decided yet. I really don't get on with Hooper much after the '70s. Unless something I haven't yet seen turns up on Netflix or Prime, probably a rewatch of The Texas Chain Saw Massacre or Eaten Alive

Extra Credit:

The Witch Who Came From the Sea.
Cube 2: Hypercube (note: there is a near-zero chance of me watching Cube 2 as I watched it just a few years ago and did not care for it).

Updates: after compiling this list, I discovered one of the intents of HoopTober was to discover new films through the categories, so there are a few substitutions and omissions.

Included Antiviral as both a Body Horror and Disease-Based Film, so dropped 28 Weeks Later as a Disease-Based choice and substituted Child's Play 2 in its place as a second film of a franchise.

Watched Saw, misremembering it as a film set entirely inside one location, so watched The Boat as well.

Watched Prey (AKA Ropes) on a whim, randomly qualifying as 6 Countries (Spain), 3 Disease-Based Films, and a bonus Film Set Entirely Inside One Location!

Watched Aquaslash from my Watchlist as it turned up on Prime, and that's another Film Set Entirely Inside One Location - I've done better finding films in this category by accident than by design!

Rather than rewatch Night of the Living Dead, I chose Larry Cohen's Bone for my second Film With a Black Director or Predominantly Black Cast or Lead.

In an attempt to watch something new, my One Tobe Hooper Film was Crocodile, picked up cheaply from eBay.

Watched Wicker instead of The Vanishing as one of my Six Countries, again accidentally adding another Film Set Entirely Inside One Location to the list.

Watched Color Out of Space as it just turned up on Prime and I could not resist. Bonus Body Horror/Disease-Based Film?

Watched The Scooby-Doo Project (because who wouldn't?), and then Rob Zombie's Halloween as a precursor to his sequel.

So, I didn't manage [REC], 28 Weeks Later, or The Vanishing, but I did make substitutions in all relevant categories, and a few additions on top. My eventual HoopTober7.0 Watchlist is here.