It already is "illegal". If they decided to enforce it then I don't think anything would really happen. It would be impossible to enforce (like stopping piracy). But even if they managed to do it somehow, it wouldn't impact Nintendo much at all.
In general, people who make fan-art / fan-games and rip sprites / sounds, etc. only do it to games they like, not the other way around. (What would be the point in making fan art of Link if you hated the Zelda series (or if you hated Link)? You don't like a game because you make fan-art, you make fan art because you like the game)
And if you like the game you will continue to buy / play it regardless of whether or not you are allowed to make fan-art of it (unless you're boycotting it or soemthing, but not buying a game because the maker of the game doesn't want you using their concepts in your art is a bad reason to be boycotting it).
Additionally, not all fans of a game are involved with fan-games or fan-art (a majority probably aren't). As a matter of fact, a majority of the customers probably aren't even "fans" (as in, they aren't members of any message boards like this or anything). So even if every fan involved with fan-art were to boycott the games then it probably wouldn't hurt Nintendo much (and if they managaed to sue a few people for thousands of dollars then it somewhat makes up for any losses).
As for message boards like this one... I'm not really sure what would happen. They'd probably just become game-discussion / non-fan indie-game boards