[Crazy Jetty]

Personally, I feel they need to tighten up the cast, and go back to characters/actors carrying over, than the throwaway casts started in X3.
I know there are a *lot* of X-Men, and everyone wants to see their favorites, but a tight core cast of generally popular characters, with two or so *guest star" x-men I think is a good formula.
Let the villain's team be more fluid. (As bad guys get killed/arrested, it's more logical that Magneto/Whomever would go through them like candy)

Sadly, the most popular charcaters are already in use, many years after this movie will be set. Assuming they're still playing with the pretense that these movies will tie into X-Men 1 and 2, then let's see Beast and Havok carry forward. They're the most logical. I think Banshee carries potential, as well.
Since Mystique and Angel left, we need another girl. With Havok on (And potentially leading the team), then Lorna Dane makes a lot of sense. It would be interesting to explore her magnetic powers, verses Magneto's.
Also, other fairly logical choices would/could be Quicksilver and Scarlett Witch (Playing simular role to Rogue, or Kitty in the first three, as the very young rookies).
If you dump the convoluted mess that adhearing to his origin as a timetraveller, and make Bishop a contemporary mutant (I think he's a great character, and explaining him as a time traveller would be too much for a movie), he would be a good choice.
There's also Sage. It seems like the Hellfire Club has been taken down, perhaps she'll be able to drop her Black Queen persona.
Sunfire is another possibility.

Then, following the theme of "The First Class," and opening the door as a full on school for mutants, the next film could easily be "X-Men: The New Mutants," and bringing in a bunch of the NM characters as pupils of Xavier, and Moira (While Havok leads the actual X-Men).

For the 'Nam question, I don't know of any vietnam war X-Men stories off hand. But they could easily set Forge in that setting. Banshee is of the age that I believe he'd get drafted. They could easily make a story out of that.