Concerning Fangame design as a whole.

[Evil Yoshi Toes]

As long as a fan game is not an exact remake of a game being sold currently I don't think it should be an issue, even though it technically could be.

Action is rarely taken when it comes to fan art, even when it is sold at conventions. With fan games it becomes trickier since it is the same medium as the official product, but unless the game is just a remake there shouldn't be any harm in it since the player is not experiencing the actual official Nintendo games. Games that I'm aware of that have gotten into trouble are Mario Maker remakes that are almost exact copies but with more features, as that'd effect Mario Maker sales, and that Mario 64 remake because it was just a remake but with modernized graphics.

Selling Nintendo fan games tends to be a harder "No!" than selling Nintendo fan art even though both are illegal since, again, fan games are the same medium as the official product. For this reason selling fan comics of an existing copyrighted comic would probably not go over well either.

So, I think fan games and art are a great way to promote a franchise as long as they are not competing enough with official products that the loss of revenue outweighs the benefits of the advertisement.