Jack must make a comeback. Yeah it's Moe, but good Moe.
Garrus, the Undying, will of course be back.
Mordin can come back, but he's kinda old for his people isn't he?
Tali, I would like to see Tali come back as an Admiral, that would be cool.
Liara should make a short appearance with a child from Shep. Leading to an awkward conversation with Jack.
Thane should make an appearance from his deathbed with some philosophy for us.
The rest I don't really care about.
A spacebattle would be cool.
Fewer cliche's, the story is kind of a homage to sci-fi, but the Bad Guys being the Old Good Guys is boring, overdone and silly.