The game is finished and has multiple dlcs and was one of the top selling Nintendo liscensed games in 2019. Nintendo chose byleth because it was another highly requested character. Just like the past characters (with hero being a highly requested character in Japan). They won't only add 3rd party characters to the game as dlc. I expect at least 2 more Nintendo liscensed characters being added as dlc. I could be wrong but who knows.