If there will ever be another faction war (which is HIGHLY unlikely, after cross-faction gameplay for raids and dungeons), then they would rejoin the Alliance. The Forsaken as a whole would probably rejoin the Alliance, since many in their leadership have close connections in the Alliance. Calia's best friend is Anduin, Derek's sister is the Lord-Admiral of Kul Tiras, Voss' best friend was a former Kul Tiran whose family still lives there. I can't imagine them ever wanting to wage war on Stormwind and Kul Tiras.
So perhaps a better question to ask would be: "Do you think the Forsaken will ever rejoin the Alliance?"
Even the term "rejoin" would be correct, since as you will recall, the Forsaken are actually the original founders of the Alliance
of Lordaeron. It would be like a homecoming.
Regardless, if it ever comes to war, the rangers who died in the defense of Teldrassil will not wage war on their own people again.
But since we are heading into a future where faction restrictions don't matter anymore, Delaryn and co. will probably remain in the Ruins of Lordaeron but keep close ties with the Alliance's Night elves. Maybe the Desolate Council will also allow Night elf families to visit the Ruins and reconnect with the Dark Rangers for some time. Just like Theron allowed High elves to perform pilgrimages in the Sunwell.