I think he's going to be changed, but not to some major degree like Anders seemed to be. It's not like he's possessed by a spirit of justice or anything. Bioware writers have said they don't bring companions back unless they feel that they can do more with their story and I feel there's a lot of room for Varric to grow and be awesome. Fundamentally, Varric was always a good person and I feel that's going to be his big flaw in Inquisition. Look at the demo and you can see him disagree with the Inquisitor's decision to leave Crestwood to its fate. I think he won't be able to think in terms of the greater good or long-term effects of his actions. But I also think he's going to be a total bro to your Inquisitor.
Because of how DA2 played out with the dlc/expansion getting cancelled, Varric is probably going to be the big source of exposition on what Hawke was up to before he/she vanished and that's probably his reason for staying with Cassandra. Given the inevitable cameo appearances by old companions, I totally can't wait to hear him say "Long time no see, Daisy/Junior/Sunshine."