Currently, the only confirmed cast member is Ackles as Dean Winchester. It's unclear if Dean will only serve as narrator on the show or make an actual appearance. Since the show is still in development, we can only speculate about who else will be joining the cast.
On Supernatural, John Winchester was portrayed as an adult by The Walking Dead's Jeffrey Dean Morgan, as a young adult by General Hospital's Matt Cohen, and as a child by Nels Guloien. Mary Winchester also made several appearances throughout Supernatural's run and was portrayed as an adult by Samantha Smith and as a young adult by Arrow's Amy Gumenick.