Thats my point. How do you know they don't have it on the table?
As it is now you are automatically assuming that you thought of it and posted it first, with NO consideration that they are working on it.
By your own arguement, you can't know what they will have, and if they start it and have it set up then you post it independantly, all of a sudden they should cancel their plans or give you credit for something you didn't do.
Prefaced with IANAL:
....if I post something to Gleemax, there is a record of my post. If someone who sees it profits from it, they profit from my creative work.
If I talk with someone behind closed doors, there is no public record of my conversation. There may be a private record, but the point is that nobody KNOWS what I'm talking about outside of those who can't really talk about it. If someone comes up with the exact same thing, they did it independently, or due to an internal leak.
If they came up with it independently, it's first come first serve. If they have record of doing something with it before I do, they win the rights to it. If I do something with it before they do, I have the rights to it.
If they came up with it due to a leak, I can punish the leaker and perhaps challenge their rights to it.
If I post something to the internet before a book containing the same thing, independently developed, I've "published" it first, I have a public record of use before WotC's book came out, and thus, I have the rights to it, and WotC is infringing upon my rights.
Do you know much about Copyright law? Because though I'm not fully trained in it by any means, I have a pretty good layman's understanding of it, and by my understanding, the law already handles cases like this just fine. Whatever problem you're seeing doesn't really exist, because such cases have come up before, and are generally settled based on who did it first.
