First let's go with whom we know will be there: Earth 2 Superman, Aquaman, Martian Manhunter and I think Connor Kent (don't know if he's official yet, but come on.)
And now onto the other incredibly likely ones.
Ted Kord - It's going to happen. If there's a "Booster Gold" tie-in it's going to have, at the very least, a zombified Ted Kord in it. It makes so much sense that it hurts...in the pants. i mean you get the automatic reaction from the character of Booster Gold, and I mean come on. The guy's been dead for a couple of years and has shown up in a number of issues of the book.
Terra (#1) - Yet again it's a huge gimmie. There is a Titans mini and you're going to need more then just Connor to make it happen if there's going to be an entire mini based on the characters. I really have little more to add on it.
A Batman - As we know, Bruce Wayne kind of died this year. And there's bound to be a Batman mini going on. Although it's kind of for the best that Bruce is dead so as we don't have to deal with zombie Thomas and Martha Wayne. They'd totally do that. I mean I would. The reason that I say A Batman is that there's going to be an Earth-2 Superman involved, I think he was burried on Earth, but Earth-2 Batman is wicked dead as well...and it'd be a cute twist.
Pa Kent - It'd be a pretty big dick move, and Johns killed him. Johns is the man running Blackest. And you don't need super powers to break someone's heart.
A Flash - Barry Allen was supposed to lead them originally. Four speedsters is just too many. Bart's back...so he's out. Barry's back...and it might be too soon to kill him. Wally is fair game, but unlikely. And Jay Garrick...really? But, at the same time there are an entire littany of dead speedsters out there so we'll see.
The New Gods - Why not? They're dead. They managed to move a pretty sub par miniseries before "Final Crisis" on name recognition alone. And they'd give the big guns some formidible enemies to hit...if they can get them out of the source wall some how. Although two Super-men may be a fairly tall order to handle independently of the New Gods. It's a thought though.
Ralph and Sue - I'd hope not actually, but it would certainly get a fan reaction. I am in the camp that they'd let them stay dead. Bwa-ha-BRAINS indeed. In all honesty they seem like almost sacred cow characters and people may react negatively to see the two all zombied up.
Max Lord, Jade, Vibe, Gypsy, Osiris, Hawk and Dove, The Girl From The Fridge, Infinity Inc. (3?), Vigilante and a myriad list of characters to long to list- Jade's a pretty obvious one. Max Lord is a probable. Vibe is a definite in my book. Gypsy is a why not. Osiris...sure. I'd imagine there's going to be a fair amount of Black Adam throughout. Hawk and Dove...falls into the sure category. Fridge Girl from Green Lantern would be an interesting...she's dead, she fits into a fridge easily...it's going to happen. The rest...probably not the recent Infinity Inc. from 52 because they just don't have that much juice to be honest with you, but you never know. Look, there's a lot out there. I'm pretty positive about the ones I've listed up top so we'll see. Hopefully it's as good as Sinestro Corps. That is all