I really don't think that there is any need or necessity to bring back Dana or Oscar Barratt for the next sequel. Those characters should be left in the past. They're not integral to the Ghostbusters story. I know I've said it before but Dana should never have been in Ghostbusters II. It was a terrible idea and adding a baby to the story just made it even worse.
The only characters I want back are Stantz, Spengler, Zeddemore, Venkman and Melnitz. I want to see a movie concentrating on these characters moving through a story which has some originality to it. There should of course be a new paranormal threat and some kind of human problem that the Ghostbusters have to deal with which is just as problematic as the supernatural occurance that they have on their hands.