I think the point of not needing new squadmates beyond the ME 1 & 2 characters (those that survived) is pretty valid. I like the idea of having your squad be more optional, but I would still like to see all of the old squadmates in some form. Maybe they could all come back (with a few exceptions) and those that don't interest the player as keeping for squadmate purposes can become members of the crew instead. This way we could still talk to these characters but will not have them in the position of more in-depth squadmates.
I think they will bring back any romanceable squadmates, or atleast have each as optional except for the one(s) you actually romanced. Wrex should probably stay on Tuchanka. I think it may be okay if they let Thane die (though I would rather they didn't) but only if you get to see him before it happens. I think it would be a good reminder that heroes die from non-violent causes as well. Perhaps he only survives if he was romanced, or atleast loyal. Samara may have her own things to work on.
Every other member should atleast come back in some capacity.
I do think there should be options for a few new squadmates, though.
I think a Batarian squadmate is the most easily realized one, it would create some interesting interaction dynamics (due to Batarians hating Humans for colonizing systems that they were already hoping to colonize themselves), and we really know very little about Batarians from ME 1 & 2. A volus, an elcor, or a hanar would be really good to add to the crew. Would be able to interact with them, but they may not be best for taking on away missions (though from what Zaheed said, Hanar can be pretty dangerous). I feel like there are enough humans on the team already, so don't really need any more of them.
I'm suspicious that Joker may be a squadmate instead of just crew in the ME 3.
I like the Rachni, but i'm not sure they should be playable. Maybe a female turian.
i'm sure there are many more ideas but i just wanted to present the most reasonable. Whomever they choose to bring back or add, however, i want to see a lot more character interaction (both with Shepard and other characters on the crew/squad.