It will happen, or it won't. It's an idea that's been bandied about. It's certainly a bug in the devs' ears by now.
If they go through with it, it's the sort of thing that will get an extra heavy development and testing cycle, like Luke and Rey did.
And dark side just got two huge freebies in the Palpatine and Vader reworks, that turned two characters a large portion of the player base already had in their rosters at seven stars and well geared into two of the best characters in the game. And even as a three-star marquee character, Sion is a huge boon to that team. And recently Matt the Radar Technician became super accessible and a huge boost to First Order teams. So dark side has been getting some love.
Lord Vader is probably on the docket somewhere. If I had to make a bet? He's slated somewhere in the year-and-a-half down time between Solo and Episode 9, which they're probably also saving Snoke and Hermit Luke for. Probably toward the end of that window, since Sith just got two major reworks and three new characters, and are currently a massive and defining presence in the meta.