I'd say the removal of survival as the more magic focused ranged spec for hunters also speaks in favor of this. Personally i'd love to see a dark ranger class but i have a hard time imagining 1-2 more specs for it• Of all the directions they could take Alleria's story, they decide to make her involved with The Void. And then she joins the Alliance. It's almost like they want to add Dark Ranger, but had no representative leader on the Alliance side... so they shoe-horned that in.
• The Windrunner sisters are meeting amongst themselves regardless of faction allegiance. This can allow Blizzard to create a single starting area for both faction Dark Rangers, sanctuary-style, where both Alleria and Sylvanas train their Dark Ranger apprentices. This would be similar to the other hero class starting experiences.
• Dark Ranger teased in Hunter order hall with the hidden quest.
• Dark Ranger teased in Seething Shore by Nathanos.
• We only have 2 mail classes right now.
• We only have 1 class that uses Bows, Guns and Crossbows.
• Dark Ranger is arguably the most requested remaining hero class from WC3 after Demon Hunter - which was a huge success.
• Blizzard originally looked like they were going to add Dark Ranger style abilities to Marksmanship Hunter: 'Dark Ranger', 'Exotic Munitions' and other magical arrows for example - but were later removed. They also temporarily removed Arcane Shot on the BFA beta because I think they are intending to anyway because the Dark Ranger is currently in the works right now. I predict they will remove it in the next expansion after BFA when Dark Rangers are added, similar to how Demonology Warlocks were changed to give clear thematic room for the Demon Hunter (Meta).
• I believe the interactions we'll have with Vol'jin (and I don't know anything about BFA content story wise yet) - to allow for a more troll-like Shadow Hunter themed spec. The other two will likely be themed around Sylvanas and Alleria. Possibly even a Warden, Shadow of Vengeance melee spec.
• The lore events in Stormheim appear to be building up to a new round of Dark Rangers - possibly explaining how the other races could be inducted into the ranks.
• The next expansion appears to very likely be Shadow or Old God themed, and we're getting hints the LK may return. If Legion was BC 2.0, the next expansion after BFA might be Wrath 2.0.