I can see where you're coming from. Midna was one of the best characters I've ever seen in a game, and certainly the best in any Zelda game. But part of that greatness is from what happens at the end, and bringing her back would kind of reek of saturday-morning-cartoon syndrome. Midna had a basically perfect character arc, and anything you add to that could only tarnish it.
I guess they could go either way with it. There's more that could be done with these characters, but also nothing that desperately needs tying up, I think.
Do you mean the group of grinning white-eyed Link clones? I'm pretty sure they were just symbolic. They looked like Link to show how he's not so different from them, and how he could be corrupted just as easily.
Creepy-ass scene, and quite a fine piece of storytelling...