Copyright protection? Hopefully, you have something proprietary. But whether you can claim copyright in your contribution depends on how "separable" and protectable your work is from the board game. If copyright exists in the expansion, then you'll have a basis for proceeding against the owners if your creative work is hijacked. (You can seek to register your claim to copyright but would need to disclaim anything that pertains to the existing game.)
Secrecy? Ideally, you would get the game owners to sign a nondisclosure, or evaluation agreement. But many toy and game makers are hesitant to sign such agreements, or if they do sign, they often seek to exclude any ideas you have that are similar to ideas they are developing. Worse, they toss out secrecy agreements and ask you to sign a waiver agreement, waiving any obligations to maintain secrecy. In that case, you have to proceed with your own personal radar and determine whether the potential risk (losing your idea) is worth the potential reward (an acquisition or licensing payment for your idea). In general, case law about submitting ideas indicates that the best you can do to shore up your position when submitting an idea is:
- maintain it with secrecy (due to the vagaries of trade secret law, the idea may qualify as secret),
- don’t submit it until the company actively solicits the idea and it is clear that the arrangement is for compensation, and
- if possible, as mentioned above, use an evaluation (or option) agreement to maintain secrecy and to demonstrate solicitation.