Legal issues with using player ideas for champions? (serious question)

I am just wondering if by posting in the forums a "champion idea" if we are signing a sort of hidden contract telling riot they can use our ideas, and if riot uses those ideas do they have to give credit to the OP or can they just freely use the ideas even if it is 100% taken and no credit given. I am only asking this because if I was to post an idea here then I have no claims to that because I just want to see it in game. It would be cool to get credit but not necessary for me. I just don't want some idiot to try to sue Riot for using his/her idea and not giving them credit because they posted it for riot to use. I am just wondering what legal issues they have to go through to use ideas from non-employees.