Personally I thought Midna was a great character and I'd love to see her come back if it were possible. She was a heck of a lot more developed as a character, and frankly, I was glad to see that Zelda wasn't the main focus as she is in almost every single Zelda game.
Now I know some will say "But the game's name is Legend of Zelda, not Legend of Midna" Well here's a news flash, personally that title should have always been Legend of LINK since he's the friggin legendary hero in the first place. How did Zelda's name get tacked on instead? I don't know... she has a good publicist and lots of money I guess.
Another note to make... I already read somewhere here that some were upset that Zelda wasn't the main focus... but they forget that this particular installment is called the Twilight Princess... something that Zelda could obviously NOT be considering where she resides and her nature as a "sage" (maybe not officially one of the 6 sages, but she certainly can be considered one) of light. So It was perfect that it was Midna, and not Zelda who was the Twilight Princess.
But I digress... back to the main point, I think that it would be great if Midna came back again. While we're at it, it would be nice also if Ganon would somehow be changed. Yes, I know, Ganon is the ultimate baddie for Link. He's like what the Joker is to Batman. But that being said, does Batman ALWAYS go after Joker in EVERY movie? No of course not! We'd get sick of the Batman movies if that's all we ever got to see. I think some of us would eventually tire of seeing Ganon in every game. Which is one of the reasons I liked Majora's mask so much. Ganon was instead replaced by a playful and devilish spirit contained in a mask and posessing whoever so wore the mask.