Just saw the trailer. Sa-weeeet!
I'm not sure guesses about squad members are useful. As *any* amount of time could have passed (it's obvious that reapers are slower than mass relays, since they use them to travel, just as humans are slower than cars), so who knows what's happened to the crew. It wasn't likely that any crew would leave at the end of ME1, but things happen between games. What I'd like is a solid mix 33% ME1, 33% ME2, and 33% new squad members. I really hated not being able to interact with my love interest regularly as Shepard, and so I hope they bring Ashley/Kaiden back, or at least allow you to visit them at some location.
The Trailer seems to explicitly tell me that the storyline is assembling a team to take down the reapers. This means, in short, that new characters will be a big part of the formula, seeing as how few of the known squadmates have much to contribute, based on what we know now. Perhaps, like Wrex, they've risen to places of influence and can bring armies, but beyond that... "I need to take down the Reapers, lets go find Thane..." Also, because its a video game meant to work as a starting point, you can count on having no squadmates on the tutorial level, at least.
Another fun thing could be *temporary* or *local* squadmates that are only available on given planets. That could be cool.
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