If you are writing an adventure, a game supplement, or any other written work based on Basic Fantasy RPG, then you have basically three options and must choose just one.
If your work is to be part of our project, that is, submitted through our forum and published by me in our usual places, submit it on the forum and we'll attend to everything for you.
If you are creating a work you plan to publish independently, and your work is "derivative," that is, you use any material from our books, you are required to publish your book under the same license, CC BY-SA, and comply with the requirements thereof. You'd have to reproduce our copyright information at a minimum, but you would put your copyright on "top." You might write this as:
If you are creating a work you plan to publish independently, and your work is NOT "derivative," that is, you don't include any of our material, you may use whatever license or terms you prefer.My Cool New Adventure Copyright (C) My Name Here. All Rights Reserved.
Based upon the Basic Fantasy Role-Playing Game, 4th Edition, Copyright (C) 2006-2024 Chris Gonnerman. All Rights Reserved.
(followed by text explaining the CC BY-SA license, similar in function to our title page text.)
The third option is tricky. You can easily write an adventure for BFRPG that does not include any textual material from our books; indeed, under fair use doctrine you probably can use names and statistics of monsters, etc. pretty directly without infringing our copyright. There isn't a lot of "artistic presentation" in our books that would open the door for infringement. And with the 4th Edition we don't have a "product identity" license because "product identity" is only a real thing under the OGL, so you can fairly freely just say "Compatible with Basic Fantasy RPG" right on the cover. In order to do this third option, you simply avoid copying any substantial text from our books.
I do recommend joining the Project and publishing through us, but it is of course your choice.
If you do not publish through this forum as part of the Basic Fantasy Project, and your work is not available for free, you may only discuss it in the Product Promotion thread here and may not promote it in the Basic Fantasy RPG Facebook group (as commercial products are off topic there).
Also: If you use any version of my "crenelated border" on your cover and don't publish through the Project, I will be cross. Consider that part of my "artistic presentation" and steer clear of it.