Well... the issue of square soft characters has in fact been brought up many, many times. Anytime that you're surprised that your search didn't bring anything up probably means that you just couldn't find it and it's there, as the forums have pretty much talked about every known and obscure video games ever.
So, the thing is, Square Enix is a third party company, meaning that arrangements would have to be worked out in between Nintendo and Square. While I think a lot of 3rd party characters would say "Oh, man, of course you can put our stuff in your game, that's absolutely great advertising for us!" it's still something that needs to be worked out.
But... still. It'd be cool, I rather enjoyed the feel of the pre-FF7 games. I don't think I'd like to see them in sprite form, however- I'd love to see the characters brought to full 3D form based on that FF artist's stylish sketches. The actual graphics of final fantasy weren't too iconic, and would just seem more impressive updated... except with maybe the very first FF games. If all the FF1 characters jumped onto the stage 8-bit style and started tearing up the place... awesome.
