Even CoD : Black Ops 2 had choices and consequences ( for a CoD game it was even impressive. For any other game it wasn't :p ) where one thing you decided to do at one point in the game could turn against you way later in the campaign ( way later being, erm, no more than 6 hours since the game is hardly any longer ). There even were multiple endings. In a Call of Duty game. yes, shocking!
And yes since the MP mode implies you start the game with 4 chars meant to be played by a specific player, I doubt there will be any chance of swapping during the course of the game. Meeting the other escapees is a possibility, but I strongly doubt they will be able to join the party. They will at best act as NPCs. Just like in Seiken Densetsu 3, where you would occasionally meet the characters you didn't pick doing their own quests or just chilling.
And while I'm aiming at playing the game coop, I am not convinced by drop-in drop-out either. I mean, either it means you're dropping in the game of someone else and potentially "ruining it", or the friend you decided to play the full campaign will somehow continue his session without you, which sounds a bit moronic =) Fortunately, if someone doesn't want his game ruined by undesired drop-ins, he should be able to create his game as solo/private/something. Dunno how it went for DOS1 in this regard.
The Brotherhood of norD is love, the Brotherhood of norD is life.
