A couple of things I would like to point out.
No one ever said yes or no to Sethrak joining the Horde… eventually. It could still happen. It is also (though far less likely) for them to join The Alliance. The Sethrak you speak of (The Faithless), I am not sure would be welcomed in The Horde, with the recent addition of the Vulpera. Who you know… were enslaved and tortured by the Faithless Sethrak.
Also, Shadowlands spoiler below that could pertain to this general topic if you are interested.
Spoilers
There is a Sethrak character in Shadowlands. I would gander she might be pretty important since she is one of the Soulbinds.And another thing. If you’re really that invested in Sethrak, (I am) maybe add an in game suggestion. It’s under the Help Tab. Help-> Submit Suggestion. 
I and other Sethrak fans would appreciate your support.
You can chat with us here on the Megathread.