I believe this topic has been covered here a number of times (have a look around the forums and you'll probably see it), but as for original costumes, most people I know seem to think it's a bit iffy. Anime and manga characters are made
to be easily recognisable (as in, they always wear the same outfits and have rather original hairdos), and it's normally hard to change something in that character design and still be recognised as that character. It of course depends, though, on how much you change and how.
Personally, I don't care - if people want to cosplay, they should, regardless of character or modifications of character, but you should be prepared to get lots of questioning looks if altering an already known character design.