Many of such instances would be handled on a case by case basis.  We got this question a bit last year as well, but in different forms:

A few quick, hard answers on some of the more common things:

Gore itself permitted when it is explicitly warned for.

Depictions of suicide are allowed when it is explicitly warned for.  Mentions of suicide are allowed, and we encourage warning for it.  Please distinguish between mentions and depictions in your warnings.  Encouragement or glorification of suicide is strongly discouraged.

Sexually explicit content is expressly forbidden from the entries as per the rules.  Anything as an extension of that would follow as well.  You can update your entries at a later point to include it.

Anything that could be constituted as hateful messaging or imagery.  Things that encourage racism, sexism or other discrimination against peoples.  

Harassment or encouraged harassment of real peoples.

This list is not exhaustive.  I would urge entrants to exercise good common sense regarding the content they enter into the contest.

Edit:

And, of course, feel free to ask ahead of time about your project/scene ideas either here or in our Discord.

> Is there a minimum required length for submissions or minimum number of assets?

There is no minimum or maximum word count or assets.  If you want to go minimalistic or all out, that's fine by us.

> Is a choice based storyline required or would a linear submission be acceptable?

Both very acceptable.  

>Would it potentially be acceptable for the game to contain a few mini stories rather than a single narrative?

That's a pretty cool idea!  Kind of like a 'Campfire Ghost Stories' thing where more than one story gets told.  That's acceptable, exciting to think about even!

> I have a vague idea of what I'd like to do but would like to plan a tiny bit ahead to organise if I need a team and what I might need to sort out before the start. Thanks!

Good luck!  If you find yourself in need of a team, make sure you join our Discord.  We have two big team building events on the 21st and 28th for anyone looking for teams that hasn't found one yet.

You're welcome to submit your Spooktober entries to other jams.

If you are making an entry for another jam, it will only be valid if it meets all of Spooktober's criteria, including limiting your work period to our jam period.

In the instance you are making a Halloween DLC style entry of your existing visual novel, you may only use backgrounds and UI elements from the original if they have Spooktober VN Jam themed elements added to them(e.g, Halloween decorations, horror genre props, etc,) if they don't already have them. 

You are not permitted to bring over music, sfx, or sprites, or any other unmentioned assets, unless they fall into the realm of publicly available resources rules as listed on the jam page.

For sprites, you're allowed to use the original sprites as a concept for new sprites. In the interest of fairness in the jam, you'll need to recreate them.  For purposes of judging, I will say the higher the deviation from the original sprites, while still being recognizable as the characters, the better.  New outfits and/or costumes are a good start.

For music, themed remixes of your originals are an acceptable replacement for the original music.

Code, especially for minigames or non-visual novel mechanics, is handled on a case by case basis.  If you have code you'd like to bring over, please discuss it with us first.  Thus far, we've permitted the use of RPG-like code from a RenPy made original game on the basis that much of its code would be available in another VN-capable engine like RPG Maker out of the box.  As a general rule, code that would normally be available out of the box in another engine is most likely okay, but we need to be made aware of it.  For judging purposes, I will say entirely new mechanics is favorable to reused ones.

We'll add a link to this question on the page for the rules regarding DLC/Spinoffs.

Technically only the project lead (who I assume would be you) needs an itch account to be able to enter and submit to any game jam. Should your friend make an account themselves, you should be able to add them as a Contributor on the game following these steps:

1. Locate the game on your dashboard and click Edit
2. In the More tab to the right, click the dropdown and select Admins
3. Enter in their Username or Profile URL at the bottom and hit the Add Admin button
4. Send the URL you receive to your teammate so they can confirm

You may! Third party assets are allowed (not just CC, but also paid or otherwise licensable) if they re publicly available to everyone to use or purchase.  

Quick note for everyone reading: The spirit of this is a level playing field for participants, so while it's allowable by the letter of the rule, some obvious loop holes are frowned upon. One such example of such a loop hole is making your own assets CC just prior to the jam starting and saying they're publicly available on your own website.

Depictions of suicide are allowed when it is explicitly warned for. Mentions of suicide are allowed, and we encourage warning for it. Please distinguish between mentions and depictions in your warnings. Encouragement or glorification of suicide is strongly discouraged.

I've also added this statement to the above thread where we cover some other sensitive material.  

https://itch.io/post/4145704

Yes, you can use commercially available assets and asset generators.

You may submit one project as many times as you want.  The very last submission you make before the deadline is the one that will be reviewed.