The most important question that any writer faces with regards to plot is conflict - resolution. Once all the problems have been resolved, the story ends about five minutes later.
So what we have to do is look at each character and what possible conflicts they might have to further their development:
Tali - still a lot of conflict here. Quarians haven't returned to their homeworld, after all. Her father died because of wanting to fulfil his promise to her - it's only fitting that you get to see that promise fulfilled. Squad member.
Garrus - I really can't think of anything pressing for him to do that would greatly advance the plot. Expect him to be a cameo (perhaps cameos will be a case of when you visit a particular world, that character joins you for that mission? Like a loyalty mission, but grander in scope). If he survives ME2, I would guess that he will leave at the beginning of ME3. Then again, due to fan backlash, he will most likely make it in as a squadie with a new conflict written for him (possibly resulting in him telling the Council and C-Sec to shove it).*
Moridin - again, nothing pressing for him to do. However, I can see him being a non squad member that still stays aboard the ship (like Dr.Chakwas or Kelly). That way, if you kept him alive, you can have the option of reversing the genophage (if you decided to keep the research), which would allow you to recruit the Krogan for the final battle - which means that you can actually get the really, REALLY good ending where Shepherd doesn't sacrifice him/herself. (Just throwing ideas out there). And you can still have conversations with him (talk about the previous mission/what's happening on the ship). But he isn't ESSENTIAL to the story.
Jacob - why exactly did he leave the Alliance and join Cerberus? The reasons he gives aren't very good ones. Plus his loyalty mission seemed contrived to me (it wasn't like he kept going on about how his father went missing before it happened...or maybe I just missed it). Perhaps they were holding something back? Anyway, he'll be back as a Squad Member, if only to balance out the presence of...
Miranda - this one still has conflict. Still has her father chasing her, and we still don't know why he created her in the first place - except for some vague thing about a 'legacy'. Plus, unless the Normandy blows up again, her place on the ship is called 'Miranda's Office'. So expect her to return to active duty. Squad member.
Samara - WILL be replaced by Liara (the reason why she couldn't be romanced, so players couldn't form an attachment to her). Doubt she'll even be given a cameo, as her involvement was merely as a placeholder.
Legion - plot development central with this guy. What are his motivations? Why find Sheperd now, and not help in ME1? Squad member.
Jack - any development of this character would be forced, IMO. If they had made it so the head researcher of the Cerberus facility was still alive, then maybe we would have something. But they didn't, so we don't. Will leave at the start of the game if she survived (like Garrus).*
Thane - will be dying. They wrote that into the story so they didn't have to worry about keeping him around (his story would have worked just as well if he WASN'T dying, which means it was a device to write him out). Leaves if he isn't already dead.*
Grunt - I guess his desire for battle is as big a conflict as you can get. It will either be him or Wrex as a squad member.
Zaaed - DLC means that not everyone will have got him, so he is not canon and therefore not relevant.
*Of course, plot devices can be written for these characters if enough people want them back.
So of the ME2 characters, we've got Tali, Legion, Grunt, Jacob and Miranda (and most likely Garrus) returning to active duty. If they had wanted the others to return, they would have given them other plot points to resolve. Liara and Kaidan/Ashley will be in, bringing our total squad to 8.
Tali
Legion
Grunt (or Wrex)
Jacob
Miranda
Garrus
Ashley/Kaidan
Liara
Leaving room for 2 more new characters.
"But what if you let your entire squad die?" I hear you ask. Well, if that's the case, tough. You'll have to make do with Ashley/Kaidan, Liara, Legion (it's a program, and therefore can't 'die') and the two new characters. Which is 5 - one less character to play with than the original Mass Effect. Remember, dying is BAD.
Of course, if you were that incompetent, I would say your chances of actually having a victory over the Reapers are going to be buckleys and none - which is why the choices you make in ME2 will have a great affect on ME3.