This is something that could happen especially the way things are going right now. Of course this a rumor and would be rather nice if it did happen.
via KiriothTV
https://www.youtube.com/watch?v=GsSSGgHWjpU
I (and others) have been informed that we may well see a new army hitting Warhammer 40,000 in 2020, in the form of Traitor Guard.
The Rumor goes like this.
Traitor Guard Coming in 2020. That's about it.