If something should go wrong regarding a Wizards of the Coast owned map that you've created and posted online, a cease and desist order from WotC, or they simply ask you to remove the map from online could be expected. Unless one is trying to sell the map, a legal action shouldn't happen at all, but the former could happen. So something to watch out for.

As Mark Oliva suggests, however, if you're creating the map for your own personal games and you do not post it online, there should be no problem at all.