Because people keep pointing out that we don't know if the classes will be the same as they were in the trilogy, which honestly had not occurred to me. I love the trilogy classes. They were symmetrical and great for my personal sense of characterization. I have some fear of change about this.
That's all I've got. I suppose I'm hoping for reassurance that they'll probably keep the old classes, or at least some educated guesses as to when we'll actually know.
The classes changed quite a bit between games.
Adepts were very different in ME 1. Vanguards have come to be synonymous with Biotic Charge, an ability that they didn't originally have. All the classes have undergone some significant changes over the course of the series.
They probably will keep the broad designations (Sentinel, Engineer etc.) for the purpose of defining roles. Given how the games have reached a point where the classes blur into one another a great deal and they all fundamentally work in any combination, the actual class makeup of the squad doesn't really matter anymore. In the original ME, I always had whichever character's electronics skill was highest with me, because their role was important. By the time ME 3 was out, you could just pick your favourite characters - it didn't matter what abilities they had, because they could all be roughly as effective as each other.
Both have advantages. If they plan to make roles more important in gameplay again, then it makes sense to go with the 6 designations that we know, and craft missions that are reactive, changing depending on who you bring with you (this takes a lot more work and resources.)
If they stick with the "pick and use whatever you want, roles don't matter and it all works," then players are free to choose companions on the basis of which ones they like the best.
With a lot of work adding tactical complexity, they can service both advantages. Because of the additional work involved, I don't expect that they'll make missions much more reactive than in the past. It is most likely that classes will serve primarily as the lore definitions of characters' abilities and how they work, but without impacting missions.