There are pros and cons to writing existing characters.
Pros are that you already have that base and people know their personality, abilities, etc.
The cons are that you generally kind of have to stick within a certain mold, if you will. Like, I can't go and make Mario a womanizer or give him the ability to control water (aside from FLUDD), just like I can't really give Zelda guns. I mean I can, but for a serious story it probably wouldn"t be for the best.
Either way, I usually prefer the freedom of creating my own story and characters from scratch.
Thanks Mao! I know you like to do this, too. It makes me want to start up an original work, too.
Although I am in more of a SSB mood.