Both sides of the argument really bring up good points in this thread.
On the one hand, I do really like the idea of having a small crew in the first game and having the majority of them be consistent throughout the series. It would allow you build an ongoing relationship with them and see how your actions/influence affect their character growth. It's one of the reasons why I enjoy Garrus and Tali so much. We get to see them really grow and change in response to Shepard's influence over the trilogy. Much more so than any other characters.
On the other hand, I like the idea of having new characters introduced throughout the series. It only makes sense that if the N7 protagonist of MENext is going to be going through the universe on a series of missions, that s/he will be meeting new allies as new threats arise. It also adds a nice emotional aspect when characters don't make it through the series safely. The Virmire choice is still one of my favorite Bioware scenes.
Perhaps it would be best to have a combination of both. In MENext1 (assuming it's another trilogy, let's say), have a small 5-6 person squad. One or two of them might not survive. In MENext2, 3 or 4 of the original squad members are back, plus they add in another 3-4. Again, one or two of them don't make it. Then in MENext3, have 5-6 of the past squad members joined by 2-3 new ones.
This way, we'd get a somewhat consistent squad (there would be 3-4 who are with you the whole time and another 2-3 who are in two of the games), but you would still have new faces, new storylines, and new dynamics within the squad.