Placing a work which uses idea suggested in a Stack Exchange post under a CC-BY-SA license is not required
Copyright law is clear: copyright does not in any case protect an idea or a method of achieving a result. In the US 17 USC 102(b) provides that:
(b) In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
There are similar provisions in the law of most countries Article 2 of the Berne Copyright Convention protects “literary and artistic works”, not the ideas expressed in such works.
The text of the CC-BY-SA license states in section 3.b:
In addition to the conditions in Section 3(a), if You Share Adapted Material You produce, the following conditions also apply.
- The Adapter's License You apply must be a Creative Commons license with the same License Elements, this version or later, or a BY-SA Compatible License.
Notice that this applies only to "Adapted Material"
Section 1.a of the license defines Adapted Material:
Adapted Material means material subject to Copyright and Similar Rights that is derived from or based upon the Licensed Material and in which the Licensed Material is translated, altered, arranged, transformed, or otherwise modified in a manner requiring permission under the Copyright and Similar Rights held by the Licensor.
Use of an idea is not a translation, alteration, transformation or modification of a source work.
Note further that section 2.a.2 of the CC-BY-SA license states:
For the avoidance of doubt, where Exceptions and Limitations apply to Your use, this Public License does not apply, and You do not need to comply with its terms and conditions.
This means that when an exception to copyright (such as fair use or fair dealing) permits a use, he terms and restrictions of the license do not apply. It also means that when a limitation of copyright law prevents a lawful claim of protection, the license and its terms and conditions also do not apply. Thus the license does not purport to cover protection of ideas, as copyright law dos not protect them.
This means that there is no such thing as a "copyright on a concept". Choice (a) of the four in the question would be the normal response, the finished book is under copyright to the author, until or unless s/he sells it or gives it away or licenses it. The author could choose to release the book under a CC-BY-SA license (which is essentially option (d)), but is under no obligation to do so. (And this would be very unusual.) Options (b) and (c) are not really legally possible. If the other person does enough of the work to be considered a co-author, that person would get half of all profits unless the co-authors agreed on a different split, which they may choose to do. But merely providing an idea in a typical worldbuilding.se post would not normally be enough to make the poster a co-author.
Derived works
What many find confusing in a situation of this sort is the issue of derivative works. When one work is based on another, the later work is said to be a "derivative work" and one may not create a derivative work from a work protected by copyright without permission from the current copyright holder. For example, creating a sequel to a work of fiction that uses the distinctive setting, and at least some of the distinctive characters from the source work would usually be considered a derivative work, and require permission if the original is protected by copyright.
But merely using an idea from an earlier work would not constitute creating a derivative work. In the US, 17 USC 102 (quoted above) would forbid this. In US law a derivative work is defined in 17 USC 101 as:
a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications, which, as a whole, represent an original work of authorship, is a “derivative work”.
Similar definitions apply in other countries, and in the Berne Convention.
Merely using an idea does not make a later work a derivative work. There must be detailed, distinctive, non-generic similarity of setting, character, plot, or other creative element of the original. General ideas such as "a couple coming from feuding families fall in love" is not enough.
Professional writers asked about the value of an idea for a work are apt to say things like:
Ideas are ten a penny. It is what a writer does with the idea that matters.
Specifically, author Lawrence Watt-Evans writes on his witting FAQ page:
I have this cool idea -- if I tell it to you, will you write it, and we'll split the money 50/50?
No. Ideas are cheap. I have far more than I can use; there are literally hundreds in my files that I haven't used yet.
I have this nifty idea -- has it ever been used before?
Yes. Pretty much every idea you can imagine has been used, and probably at least a decade earlier than you'd have thought possible. (The first story describing something like the Internet was published in 1909.)
So simply using ideas, particularly ideas specifically offered to help a writer in progress, will not require the resulting book to be placed under CC-BY-SA. However, if such a post contains a detailed and specific way to use an idea, copying those details might require getting permission from the post author. In practice, such a suit would be quite unlikely.
Multi-Licensing
Note also that the author of a work released under a CC-BY-SA license, such as a SE post, is free to re-release under a different, less restrictive license, if s/he so chooses. The copyright owner is not bound to continue using the CC-BY-SA license, nor to use the same license to everyone. The poster of an SE answer could, for example, grant the poster of the corresponding question a free license to use content from that post on any terms the poster chooses, including on simple attribution alone, with no share-alike requirement.
Thus if the person who posted the question and wants to use content from an answer asks the poster of the answer for permission, and gets it, there is no need to place a book under a CC-BY-SA license, even if the book uses so much of the post as to become a derivative work of the post (unlikely but possible, as described above).
Attribution
The question reads:
And don't think that creating a useful attribution from a website that uses usernames wouldn't by itself cause a legal nightmare. But let's assume I was able to get in contact with the respondent and get a suitably legal attribution.
One may validly attribute to a pen name, which is what a user name is, legally. It serves to credit the originator of the idea or text quoted. Many CC tests are posted under usernames or pen names, and people make reasonable attributions and fulfill the terms of the license often. Something like
Concept based on a post to worldbuilding.stack exchange by user "QRS" on {date}. Post available at {URL}.
ought to be a sufficient attribution. There is no need to learn the legal name of the poster. In fact all the CC licenses allow the licensor to specify a pen name to be used for attribution, and in the absence of any other statement, the user name would be that pen name.
Fair Use
The question reads:
... CC-BY-SA made that portion of the book open game for "fair use".
A CC-BY-SA does not in any way increase (or decrease) the scope of fair use in a work licensed under it. In fact, where fair use (or any other exception to copyright) applies, the CC-BY-SA license explicitly does not apply. A CC-BY-SA license is a grant of permission, over and above what fair use permits, to use the work. CC-BY-SA seems similar to fair use in that it allows a person to reuse a work, or part of one, without specifically asking permission, and without paying any fee.
But the basis is different. Under a CC-BY-SA license there is no need to ask permission because the owner has already given permission, that is what the license does. That permission is limited by the terms of the license, including the SA part.
Under a claim of fair use one is allowed to reuse a protected work because the US Congress has decided that m a limited amount of reuse is for the public benefit, and has included this exception into the rights given to the copyright owner. Any fair use is restricted by the terms of 17 USC 107 and the complex case law developed under that provision.
So each grants permission for reuse for specific reasons subject to limits. But the entity doing the granting is different, the reasons are different, and the limits are different.
CC Licenses
The questiion reads, in option "d":
(d) There is no practical copyright, the entire book is CC-BY-SA
It is not the case that a CC license, such as CC-BY-SA gives up or loses copyright. It is the case that the owner of the copyright in such a work gives up some of the rights s/he would otherwise have had, including the right to insist n payment for any use or modification of the licensed work.
However, a commercial filmmaker is unlikely to want to use a work under a CC-BY-SA, because of the license that such a film would need to be released under.
As I explain in the "Multi-Licensing" section above, the owner of such a copyright can still grant to a person or firm a different license on different terms. A filmmaker could still get the right to create a film based on such a work in a traditional arrangement in return for a royally or fixed fee, and the filmmaker would not need to release the film under a CC-BY-SA license.
