Yes, Vel'Koz is official, and he'll be released with the alternate Battlecast skin. With the reveal of Battlecast Vel'Koz, however, comes yet another possible teaser. In the official splash art, a floating figure is seen in the background. Who could it be?
Vel'Koz was first teased back in early January, when "leaked" assets depicted a purple tentacle creature from the Void. Hell, we even had his kit and spell effects. Sharp-eyed League of Legends fans then pointed out a connection to Vel in Creator Viktor's splash art. He was then officially announced this past week, almost a month after the leaked information.
Fan speculation regarding Battlecast Vel'Koz proved correct as well, as Riot revealed shortly after the champ announcement. Shortly after that came the splash art, and the League community quickly spotted a caped -- or winged -- figure floating way in the back, behind Viktor and layers of smoke.
So far, I've already heard Xerath and Aatrox put forward as possible candidates. At the moment, we can't know for sure if it's a skin for an existing champ or a new champ entirely.