I've got an expansion for a semi-popular board game that my friends and I have been having a lot of fun with. I was wondering if I could package and sell the expansion to the game without consent from the original publisher. I know I'd need to create all of my own materials (artwork, text, etc.) but I wasn't sure if I could market it as "Third party expansion to X".
Are there any pitfalls I should be wary of? I don't want to rip off the company or cause problems with them, but ideally I wouldn't need to have a lot of contact with them either (since they may not like it, even if it is perfectly legal.)