Hell I don't know, some research would be needed there. Whatever the major hubs were around that era I guess. I just love Viking lore, and they were expert sailors, so the ship combat would be a given. I think they could really revamp the combat as well, maybe have a more brutish feel to it.
I suspect they'd probably set the hub around England, since they did raid places like London, East Anglia, Mercia and Northumbria, but most of these places were seriously fucking limited in terms of assassin navigational options in the 750-850A.D. era it'd likely be set in.
I mean I guess they'd be going open world sailing like ACIV (even though viking ships were far more limited than what we saw in Black Sails), but the cities themselves would probably be super dull.
I think i would like an Assassin's Creed set in a memorable city again before something like that personally since it's been a bunch of AC games since we last had that, then we could return to the idea