I think adding Dark Link would be too much. I thought there was plenty of content in Skyward Sword. If Dark Link was in Skyward Sword I would want some sort of story to build up about him and not just fighting and not really knowing why.
It would be cool if they had put parrys (block with ur sword) in SS so then we could have an old fashioned battle with dark link, blocking, parrying and retaliating, looking for weaknesses in dark link
I'm on the fence about this. While I think it would be an awesome fight and a Dark Link with really good AI would be awesome to battle with motion controls, Dark Link has been done to hell. I think he's been used to much, and will need a few more games to let him be.
Personally, I think Ghirahim is a shadow link, although he doesn't have the skills as shadow link. I had that thought when you fight him just before "Demise" shows his true form near the end of the game.
I could be wrong, but either way, it's a wicked battle that makes for a good boss.
I didn't miss Dark Link in SS, but I would love to see him return in the next Zelda game. Would love to fight him with motion controls as well. Plus, he would look awesome if they really go for the graphics in the Wii U demo.