The way i see it:
Tali and Garrus are almost certainly going to be in 3 from the original ME as recruitable. After all, they're awesome.
Grunt is likely to go join Urdnot, or do something on his own to save complications with Wrex/Dead + Grunt/Dead storylines. Meaning we get yet another new Krogan

Thane was said to have a few months to live so is unlikely to be featured as more than a short story. Recruiting his son is unlikely.
Samara will just go back to being a Justicar in Asari space and at best will have the odd mention in galactic news.
Mordin i think is the most likely to follow over, unless they think it's just too much to expect him to have even more awesome lines and save dissapointing fans.
Legion is too varied since there were 3 hugely different ways to deal with him and his quest. If the Geth are even really in ME3...
Jack may just vanish and the stories you hear depend on your relationship with her.
Miranda and Jacob... that depends on the Cerberus element of the 3rd game. Its likely you'll meet them but not be able to recruit.
I think the problem is whether or not they can really give some depth to everything. They've left themselves such a huge task, making the 3rd story not contradict the 1st and 2nd and adding elements into them both from the first 2. Kudos for trying since it makes the game so much better, but 3 games is probably a bit much to do it. If they end up spreading everything too thin to meet deadlines its going to be a huge letdown.