It depends which characters you're asking about.
For most of the new-series companions, Russell T Davies went out of his way to provide a good ending for their stories, and Steven Moffat has directly said that he doesn't want to ruin those endings. So, we get brief cameos (like Martha, Donna, and Rose in "Let's Kill Hitler"), but we're not going to get new adventures with them—especially with Rose stuck in Pete's World, Donna unable to remember the Doctor without burning up, etc.
Unless, of course, Moffat writes a story (probably for the 50th anniversary season) that he thinks is so good it's worth changing RTD's endings. That may happen, but it's not going to happen every week.
Jack is an exception; he didn't get the same kind of ending. And they have already tried to bring him back (he was going to be one of the Doctor's allies in "A Good Man Goes to War", but the Torchwood shooting schedule prevented that), and there's no reason to think they won't try again—but of course also no guarantee it'll ever work out, either.
Compare things to the classic series. Most companions who left the show never came back again. A few came back one time for a multi-Doctor special. Sarah Jane, Jo Grant, and K9 are the only ones to appear in the new series and its spinoffs. (They also tried to get the Brig twice, but his health problems prevented it.) Bringing back a 40-year-old Ace and an 87-year-old Ian would just be a continuity-fest that gives a small number of fans a little bit of fun at the expense of driving away millions of casual viewers (and, at the same time, the really hardcore fans, who will demand to know why Ace doesn't have her time bike and Ian doesn't look like he's in his 30s). It's only worth doing if there's a good story in it. (Personally, I would love for Moffat to come up with a story that absolutely needs Ace in it so he has a good reason to bring her back… but I'd hate for him to bring her back as 'generic middle-aged woman who knows the Doctor'. And I don't think he'd ever do that.)