You say "magic related" product. Are the patented ideas? Are you hoping that they will market your idea or are you wanting to license? Licensing is probably an easier sell, but you need to have a lot of capital and be able to show a business plan. If you want them to market your ideas, then you should probably contact Marketing or Brand.
I don't think you would get very far contacting Rosewater unless you are talking about things for the game itself. Since you say "Magic related", I'm assuming that is not the case, so Brand and Marketing are better options. If you are unsure who to contact, start with customer support and they could help you contact the right people.
Don't set your expectations too high, though as they are unlikely to do much with outside ideas unless you can really sell them on why they should be interested. And definitely do something to protect yourself as DF says. The easiest way would be to seal your ideas in an envelope and take it to the post office to be certified. I don't remember the exact process, but this will demonstrate that it was your idea prior to speaking with WotC, so if they try to run with it without giving you credit (doubtful, but you never know), you can demonstrate that you gave them the idea. Ideally, you would patent the idea first, but that is a far longer and more expensive process.