Characters will most certainly remain in place into the next game. I think rather than installing 2 as an add-on to 1, they'll simply carry over content from 1 into 2. Like keeping the existing Patrols/Strikes/Raids, and adding a whole full-game worth of new stuff. If you think about it from a story perspective, they can't keep sending us back into the beginning story missions for 10 years. So maybe they'll even change what story missions happen in the old locations to fit new plot lines? Kind of hard to predict exactly what they'll do. But I doubt they'll just throw out everything they (and we) already have.