Where can you post new Dominion card rule suggestions?
If you have read the post that you link to, then you are familiar with forum.dominionstrategy.com. They have a subforum just for this sort of thing here.  If you post your cards there, people will likely comment on them, give advice, and even try them out and playtest them if they are interesting enough. It's a friendly community with lots of people who are experts at Dominion.

Donald X posts on that forum as well, and you can "contact" him there by asking questions in posts, but he has said that he does not read fan cards, largely because he wants to make sure that any ideas he uses are his own.