I feel evil, but the first role that came to mind was Ramandu.
Maybe... I dunno. I have no clue how close the film will be to the book. So, in the books, there's no radio announcer (LWW) or Telmarine crier (PC). Maybe they added something else. Who knows.

If he's to have any role in this it might be very small like the previous ones, so that rules Ramandu out probably, I'm not saying it as a fact. Probably as one of the crew once in the background, a villager on the Lone Islands. Something like that probably.

So do you think he will be someone who makes some sort of announcement? Interesting....

I wanted him to play Lord Bern. He just looks and sounds exactly how I always thought Lord Bern should. But since he's not, I expect he will probably have a small role like the previous two. But I have no idea where. At the moment I'm thinking something in England, perhaps an old soldier or a professor at Cambridge.

Maybe he is that guy at the beginning of the trailer when Edmund tries enlisting into the army. Probably not, but it makes me curious.

or maybe he'll do the albatross's voice!

I can't see him in any sort of "main" role, and by that i mean any character with extended screen time or multiple lines of dialogue. So in other words not Ramandu, and not Uncle Harold (the former i'm not too sure will even be in the movie, the latter surely has to be in the movie - i just can't see how the opening scenes of Lucy and Edmund returning home with the groceries would work without Eustace's parents being present at home). I also don't think he will be one of the three sleeping lords either, or any other named character really.

The extent of his cameo did increase slightly between LWW and PC, so i might expect a similar increase in the extent of his cameo for this film, maybe something where we can actually see him properly? There are a few possibilities i can think of. I don't think it will be anything too exciting though. And of course there is always the possibility that he won't have a cameo at all.

1. Someone in Cambridge - those opening scenes of Lucy and Edmund walking back through Cambridge provide ample opportunity for Doug to have a small cameo with a brief line of dialogue. A cambridge local? A home guard soldier? Add in the fact that the real-world based portion of the movie could be much more extensive than the small snippet we saw (we've had two extras castings for American military personnel, plus Susan and Peter who have to appear somewhere) and the possibilities are almost endless.

2. Someone in Narrowhaven - again, its a big bustling city. There is plenty of opportunity for a brief cameo as a local Lone Islander or one of Gumpas' guards, or some other minor, but significant, background role.

3. A Dufflepud Voice - Doug's cameo's in the last two films were essentially voice parts (you can barely see his town crier in Prince Caspian, even if you pause the DVD and zoom in). So perhaps they might continue in that tradition and have him as one of the invisible Dufflepud voices? "That it is, Chief, that it is"

4. Ambassador at a Party - Remember a while back there was a news story related to the casting of a woman as an "Ambassador's Wife a Party?". We still have to clue as to where this party might be set (somewhere in the real world, or perhaps on the Lone Islands) but perhaps Doug could play a fellow dignitary at this party?

Yes I agree with Icarus. He may end up doing only another voice over. It makes me think about a Dufflepud, a bird at Ramandu's Island, etc or a call that breaks out the fight at Narrowhaven.

or maybe he'll do the albatross's voice!

Ah, good idea, Libby! I would love that!! He would be perfect!

I hope he gets to be a sailor on the ship.....he'd be perfect!

A sailor on the ship was one of my top theories. He's got to have some role where he calls something in a distance or another voice on the radio. He obviously doesn't like to be seen in these movies.

3. A Dufflepud Voice - Doug's cameo's in the last two films were essentially voice parts (you can barely see his town crier in Prince Caspian, even if you pause the DVD and zoom in). So perhaps they might continue in that tradition and have him as one of the invisible Dufflepud voices? "That it is, Chief, that it is"

Haha I like that, and I think you're right.

A sailor on the ship was one of my top theories. He's got to have some role where he calls something in a distance or another voice on the radio. He obviously doesn't like to be seen in these movies.

ditto
I hope we'll be able to figure it out when we hear it!

The main reason i didnt think he would be a sailor on board the ship would that you would usually expect the crew, even just the background members, to be consistently present from the start of the voyage to the end. It would perhaps look slightly odd if Doug Gresham just popped up as a sailor for one brief moment. If it were just a voice role though (perhaps, like Damsel suggests, a sailor shouting something down from the Crow's Nest) then that probably wouldn't cause any major continuity problems for the ship's crew, so that might work quite good.