Sorry if this has all been discussed before but im new here!
The lack of new squadmates does worry me a little, in one of my playthroughs both Garrus and Tali died on the Suicide Mission, not because my lack of ability! I thought it would be interesting to spice things up by not buying one of the ship enhancements which resulted in Garrus dying and Tali randomly dropped dead when I left her to hold the line despite being loyal! Much as I do like Ash/Kaiden and Liara they along with Vega provide little choice, especially if you chose Ash to survive at Virmire (Her and vega both being soldiers)
These are my theories for the rest of the squad and how they will be incorporated into ME 3
Legion: I personally believe he is almost dead certainly a squaddie. The fact we got him late on in Mass Effect 2 supports this. It is highly unlikely both he and Tali die on the SM and can see him as a good replacement for the tech expert. I think he can also easily be incorporated if you chose to hand him over to Cerberus in ME2, there never was any evidence he died...or did I miss something?
Samara: Wont be a squaddie, I dont think she will even be a temporary squaddie. I can see a cameo possibly in one of the side missions but due to her role kinda being redundant in ME3 (We got Liara back) I really cant see her playing a large role.
Morinth: Again I dont believe she will play a large role. The fact that even if you chose her over Samara she still spoke like her and there was little extra dialogue makes me think BW wont feature her heavily in ME3. Perhaps a reference..
Thane: Thane will be in the game somewhere but again I dont believe he will be a squaddie. The fact he is dying supports this. I do think he will feature in perhaps his own side mission or will feature in an overall mission; not sure which one!
Kasumi/ Zaeed: Cameos from these two and I dont expect they will be large, although you could get your flirt on with Kasumi I dont think she will be incorporated as an LI. Much as I love her! And the fact you could kill Zaeed in his own loyalty mission means its unlikely he will have a large role.
Grunt: Another tough one to judge, I guess his spot is kinda redundant on the squad now we have Vega and possibly Ash. I can see BW making him a squaddie but then we have a squad thats pretty heavy on firepower and not a lot else! I can definatley see a cameo on Tuchanka with or without Wrex. Im still holding him to the 'if I find a clan I will destroy you' thing.
Jack: We know shes returning but I cant see her as a squaddie. I mean sure she was popular but in terms of the overall story I dont feel she contributed all that much, I for one do like her but shes not got much reason to reunite with Shep in ME3. I can see her as a temporary squaddie perhaps in a mission against Cerberus.
Mordin: Other than the cameo on Sur'Kesh I cant see him being involved with the rest of the game, perhaps a 'stay at home' spot on the Normandy but I highly doubt him as a squad member.
Jacob: An unpopular character who i fear will get a larger role purely because he was an LI. I know people invested their time in him, and got to see his abs

But I feel a bland character like him should be pushed aside in favour of those who were more interesting. A cameo for sure , but I fear it may be more.
Miranda: The toughest one of the bunch, We know she has a large role but will she be a squaddie? Much as I would love her to be im not sure. As a temp almost certainly, her 'unfinished bussiness' with Cerberus should be interesting and can see her rejoining Shep for some good old revenge. Beyond that I think she will be ever present in the story but not as a squaddie.
Sorry for the long post but ive been dying to express my views ever since I started playing ME2! I left Garrus and Tali out because we already know they are squaddies and as such will be an integral part of the story.