My thoughts on characters from previous games in ME3
Sheppard: Probably returning to the Alliance either in the intro or around halfway through the game. Another possibility would be trying to independantly form a coalition of forces(including the Geth) to fight the Reapers, Dragon Age style.
Ashley Williams: Probably returning to full squadmate status if Sheppard returns to the Alliance. Possibly returning as a LI if Sheppard didn't court someone else in ME2. Or not, if she got killed in ME1
Liara T'soni: Not returning as a squadmate, but may be primary source of information after Sheppard ditches TIM.
Kaiden Alenko: Same as Ashley if he survived instead of her.
Garrus Valkarian: An obvious squadmate, having been in both pervious games. Possibly more comments about your supportive waist. Hopefully you won't have to earn his loyalty yet again.
Urdnot Wrex: Does not join your squad, but rather leads the combined Krogan clans against the Reapers at your request. If he survived ME1.
Tali'Zorah nar Rayya: Same boat as Garrus, minus awkard flirtation, plus debilitating alergy to everything.
Jacob Taylor: Too bland to return. Could go either to Alliance or Cerberus as an NPC.
Miranda Lawson: Possible returning squadmate. Expect a confrontation between her and Ashley if she does.
Thane Krios: Dead, obviously. Or not if Bioware decides to be lame and cure his terminal disease.
Grunt: Could go either way. His name pretty much sums up the extent of his usefulness either as a squadmate or as an NPC. Don't see much character development left in him unless you want to introduce a Krogan romance option(hey, I didn't see the Turin one coming either).
Jack: Again, could go either way. She's useful and at least more interesting than Jacob, but she also already has the perfect out, seeing as she very clearly said that she would cut and run after the mission.
Mordin Solus: Probably relegated to onboard researcher/advisor. May complete the Genophage cure that you found in his loyalty mission to help boost your chances against the Reapers if you kept it.
Samara: Gone. Her oath to you ended and she'll be on her Justicaring ways in all likelyhood.
Morinth: Same thing if you ended up with her. Assuming she didn't critically fail your mission, she'll probably get bored and run off to find other vict... lovers.
Legion: If I was to guess, I'd say that Legion will not be in your squad for ME3. Rather, I think he'll serve as a diplomatic link to the Geth. On the other hand, he is one of the few characters from ME2 that still has plenty of room for character development, so I'm not completely ruling him out.
Zaeed Massani: Contract is finished and so is this non-interactive DLC character. Goodbye and good riddance.
So by that reckoning, we would have between 5 and 7 returning squadmates, leaving room for 4 - 6 new characters if they decide to go with a similar number of characters to ME2. If I was to guess, I'd say it will be 5 or 6 old characters and 5 or 6 new characters, that way even if all of your elligible squadmates from ME2 died in the suicide mission you will be left with a number of characters roughly equal to that of ME1.
That said, it occurs to me that Ashley/Kaiden may have been relegated to NPC status in ME2 specifically so you couldn't get them killed in the suicide mission. It was made pretty clear that Ashley(I don't have a save where I kept Kaiden) was only refusing to join you because you were with Cerberus. That also makes me think that we might see a LI confrontation between Ashley/Kaiden and Tali/Garrus that brings up the point that Ashley/Kaiden was more loyal to the Alliance than they were to Sheppard.