There are two directions a question like this can go:

1. What features of a game make it a good one to run on Halloween?

This is the more abstract version of the question. The resulting answers will probably contain bits and pieces of a lot of different settings/systems, rather than a concrete "play this" kind of deal.

2. What's a good one off game to run for Halloween?

This would be under the system-recommendations tag, and should follow the usual guidelines.

I'm a little unclear as to what you're asking exactly, but I'll try to give you my take on the individual questions... Let me know if I seem to be missing something.

[...] would that be closed as off topic because it is too broad?

Adding "for Halloween" isn't going to change the on- or off-topic nature of the question. System recommendation questions remain border-line, but generally supported. More abstract questions live or die on the quality of the expression of the question.

What if I were more interested in plots than systems, or the other way around?

Then specify as such in the question body. Perhaps include the settings or adventure tag if your question relates to those areas.

Duplicate?

I'm not quite sure what the question is here. In general, if your question is answer by another question (or would be answered by a question that isn't answered yet), it's a duplicate. If it isn't answered, then being explicit about how your question differs will help keep it open.

One final note on Halloween:

There are a lot of different feels associated with Halloween. Off the top of my head, there's soft and fuzzy (candy and jack o' lanterns hooray!); corny (and when they got home and opened the door, they found a hook hanging from the handle! Aaah!); campy (and then the teenagers stuck in the house with the slasher decided to split up...); scary (You know, I don't think I want to go to bed after that last adventure...); or celebrity guest star (ghosts, zombies, vampires, werewolves, oh my).

There are two directions a question like this can go:

1. What features of a game make it a good one to run on Halloween?

This is the more abstract version of the question. The resulting answers will probably contain bits and pieces of a lot of different settings/systems, rather than a concrete "play this" kind of deal.

2. What's a good one off game to run for Halloween?

This would be under the system-recommendations tag, and should follow the usual guidelines.

I'm a little unclear as to what you're asking exactly, but I'll try to give you my take on the individual questions... Let me know if I seem to be missing something.

[...] would that be closed as off topic because it is too broad?

Adding "for Halloween" isn't going to change the on- or off-topic nature of the question. System recommendation questions remain border-line, but generally supported. More abstract questions live or die on the quality of the expression of the question.

What if I were more interested in plots than systems, or the other way around?

Then specify as such in the question body. Perhaps include the settings or adventure tag if your question relates to those areas.

Duplicate?

I'm not quite sure what the question is here. In general, if your question is answer by another question (or would be answered by a question that isn't answered yet), it's a duplicate. If it isn't answered, then being explicit about how your question differs will help keep it open.

One final note on Halloween:

There are a lot of different feels associated with Halloween. Off the top of my head, there's soft and fuzzy (candy and jack o' lanterns hooray!); corny (and when they got home and opened the door, they found a hook hanging from the handle! Aaah!); campy (and then the teenagers stuck in the house with the slasher decided to split up...); scary (You know, I don't think I want to go to bed after that last adventure...); or celebrity guest star (ghosts, zombies, vampires, werewolves, oh my).