Not going to bother with speculation as there are plenty of people willing to do that.
I'm pretty torn as to whether I want him to return or not, regardless of role. He won't be written by David Gaider, and I dunno... somehow just knowing that makes it sad for me. That said... he is my favorite NPC and follower of all three DA games and I would love to see him and hear (Ramon Tikaram) again.
As an avid Dorian-mancer, I do wish to caution my fellow D-mancers against hoping for an excessive amount of romance content, even if the Inquisitor is in DA4 in some capacity. I really don't see them giving too much play to that specific option, especially when all of the other romance options likely won't get additional play. As Gervaise mentioned, the sending crystal might play a role, but that would apply to a friend Inquisitor as well as a romance one. So as much as I would like more romance content, I really wouldn't get my hopes up for more than a line or two, which is what we've gotten with every single other romance carryover after DAO. Probably the biggest exception was the Alistair romance scene at the start of DAA where he says a line and gives a kiss. But again, that was a very brief scene.
I don't mind return NPCs as long as it's fitting with the story and the setting. Dorian would certainly be appropriate for DA4 IF we do head to Tevinter. We shall see!