1. All included series must be a film/TV property first even if the connection is through a comic book. Hack/Slash is an obvious exception.
2. All references to other franchises has to be an intentional universe link. For example, Invasion of the Body Snatchers is shown as fictional in Gremlins, so it is not likely not an in-universe link. Grandma Sawyer in The Howling is probably not intended to connect, though the Chucky tie-in comic does help justify a link I suppose.
So, yeah. If I were to include more, it'd be:
1. Digging Up the Morrow
2. Leprechaun (what other Leprechauns would Cassie be talking about!?)
3. Night of the Demons
4. The Howling (because of the two references)
5. Gremlins
6. Explorers (references Gremlins, same director)
I'm skeptical of The Time Machine being linked, since it seems to work differently to that film. Robbie the Robot (Forbidden Planet 1956) is another interesting connection in Gremlins, but I doubt it's supposed to mean anything.
The Howling VIII is out-of-print so that's screwed. I'm gonna nab the DVD. Just in case, one day, y'know.