6 sequels (mix-and-match. 6 total) -
Survival of the Dead
Twins of Evil
Freddy's Revenge
Night of the Seagulls
Puppet Master II
Inferno
The Brides of Dracula
Bride of Re-Animator
Saw II
Halloween II
The Golem: How He Came Into the World
Bride of Frankenstein (late addition)
Friday the 13th Part II (late addition)
Friday the 13th Part III (late addition)
Friday the 13th Part IV (late addition)
The Revenge of Frankenstein (late addition)

6 countries -
USA
UK
France
Spain
Germany
Italy
South Korea
Japan
Indonesia
Turkey (late addition)

6 decades
1920s
1930s
1940s
1950s
1960s
1970s
1980s
1990s
2000s
2010s

6 films from before 1970 -
Diabolique
The Invisible Man
Mark of the Vampire
Der Golem
Jigoku
The Brides of Dracula
Dead of Night
The Innocents
Frankenstein (late addition)
Bride of Frankenstein (late addition)
Creature from the Black Lagoon (late addition)
The Wolf Man (late addition)
Cat People (late addition)
The Curse of Frankenstein (late addition)
Night of the Demon (late addition)
Invasion of the Body Snatchers (late addition)
The Blob (late addition)
The Mummy (late addition)
The Curse of the Werewolf (late addition)
White Zombie (late addition)
The Revenge of Frankenstein (late addition)

6 films from the following: Carpenter, Raimi, Whale, Browning, Craven, Tom Holland -
The Hills Have Eyes (Craven)
Last House on the Left (Craven)
Prince of Darkness (Carpenter)
The Invisible Man (Whale)
Mark of the Vampire (Browning)
The Unknown (Browning)
Frankenstein (Whale) - late addition
Bride of Frankenstein (Whale) - late addition

3 people eating people (non-zombie)
Raw
Motel Hell
The Hills Have Eyes
Society (late addition)
Split (late addition)

1 Hammer Film -
Twins of Evil
The Brides of Dracula
The Curse of Frankenstein (late addition)
The Mummy (late addition)
The Curse of the Werewolf (late addition)
The Revenge of Frankenstein (late addition)

1 Romero Dead film -
Survival of the Dead

1 terrible oversight -
Diabolique (honestly, probably a third of my list could fit here)

2 Tobe Hooper films -
Salem's Lot
Lifeforce

Extra Credit:
It (miniseries)
It (film)

For my base list I've aimed exclusively for films I haven't seen before, and I will add films to this list as Halloween approaches as my (rather young) kids will probably have a few requests for rewatches.