Think of it this way--if it would fail a plagiarism test if you were writing a novel instead of a game supplement, that's the line you don't want to cross.
Ah, but plagiarism is not the same thing as a derivative work. Or to put it differently, a work may seem like a plagiarism without even being a derivative work. Plagiarism is when you "borrow" someone else's style of expressing himself and his ideas, but neither of those are protected by copyright law.
And here I will get a bit esoteric and probably a bit OT for this board, but since questions like this intrigues me... and they do have a bit of relevance to OGL/GSL.
There's a trap here, a logical error that I've even seen courts do, and that is to mix up the underlying idea with it's expression and assume that if work B is an expression of idea A and one an only learn of A and it's properties through B (that is B is the only known expression of A) then if someone expresses the idea A in the new work C, C must then be a derivative work of B. Not true. You are free to derive from A as much as you want since A is just an idea, a concept. If A can only be studied through B, then by necessity some parts of B must also be merged with A and therefore lack copyright protection. In particular the facts about A, even though they may be fictional, lacks copyright protection.
Obviously we have now entered the realm of legal science and of course the solution to the problem at hand will be solved differently in different nations.
The point I'm trying to make is that there is no reason to accept over exaggerated claims of copyright. The case law is not so firm that one might think, fans in particular ought not feel restrained when creating and publishing their spin-offs of ideas and themes found in RPG:s.
I realise that this all sounds a bit idealistic and that in the end people make economic assessments of the risks involved and stay well out of the way of aggressive litigators. But that also means that we accept abuse of the legal system and it's exactly that kind of abuse that has created the file sharing mind set.
I can have a comic strip with an intelligent dog but if I start having him sleep on the top of a doghouse and imagine him being a veteran of World War I I'm sure the Schulz estate and United Features Syndicate would send me a warning.
Possibly but, from a theoretical and moral point of view, they are wrong. You are, and should be free to create your own version of a WW1 veteran dog sleeping on the top of a dog house.
I think it's a question of whether or not you are ripping off another's creation. Basically, you should be creating something that has your own theme. Unless it's really licensed, released under the public domain or creative commons, etc, you don't want to rip off other's work--I'm not speaking from a legal perspective, but as a creative type who would not want to just copy other's work.
We all stand on the shoulders off giants, no one is free from influences from the past and present. Creation is, really, a matter of mixing and re-mixing. Copyright law is not designed to prevent this re-mixing. While I can understand how you feel about this, from a legislative point of view, it's perfectly all right if someone rewrote D&D in new words and started to compete with WotC over the customers in the D&D market. It's OK.
As a creator I would probably be a bit mad if someone exploited my ideas but if someone else could do my thing better than me - why shouldn't they be allowed to try and see how far they could go?