As of 1pm BST Wednesday 29 April Ubisoft have officially shown off artwork of the new Assassins Creed game on Facebook and confirmed that it does indeed centre on Vikings and called Assassins Creed: Valhalla, with designer BossLogic drawing a live graphic on stream confirming as much. This became apparent around the 5 hour mark when Viking shields were drawn on the ship but has now been confirmed.
Source: N4G PC Assassins Creed: Valhalla Officially Confirmed; Trailer Coming Tomorrow