As many or all of you have seen, the most
recent timeline from the SWTOR community fansite went into detail about the Sith Empire's intelligence agents, and their role in the Mandalorian attacks. The intelligence agents are described as "Just as deadly as their Sith superiors", and their efforts lead to the creation of a new Mandalore. This manipulated Mandalore became a puppet of the Sith Empire. The video highlights the influence and power of these agents. The previous timeline video "The Mandalorian blockade is broken" followed a similar pattern. The Smuggler class was revealed around 2 weeks after it came out, and the video itself showed the importance of smugglers to the Republic's survival.
The key question here is: Is this another class hint? I would say yes. Firstly, the Imperial Agent type of class fills a clear role in the Sith side. The two revealed sith classes, Sith Warrior and Bounty hunter have clear counterparts on the Republic side (Trooper and "Jedi"), as they both fill the same role (Heavy ranged damage and Melee fighting). This leaves the smuggler as the "odd one out" of the current class pairings. The smuggler's cover system, dirty tricks, and general subterfuge need a counterpart. This is where the Imperial Agent comes in. I foresee this class as a much darker version of the Smuggler. Where the smuggler uses comical moves, including the infamous "kick", the Agent could use even dirtier tricks. I could see poisoned weapons, a sonic stunner (a miniaturized pistol capable of causing excruciating pain), and aimed shots to disable and kill enemies.

The Sith Spy also brings a unique story to the table. Each story created so far fills an iconic story or role. The Trooper fills the "war movie" role, the Bounty Hunter fills the "Mercenary" story, the Sith Warrior fills the "Dark Jedi" story, and the Smuggler fills the "Romantic war comedy". The Sith Spy's story would be a dark tale of intrigue, treachery, undercover missions, and fills the "James Bond" fantasy. I could see a variety of plot lines opening up, including infiltrating the Empire's ranks in order to root out traitors, or assassinating a Republic Senator. Bioware is attempting to create (To quote Sean Dahlberg) "8 unique classes" in terms of story as well as gameplay.
The final case for the Sith Spy comes from screenshots from the game and from the cinematic trailer. This screenshot (Credit to Alkarin for digging this one up) could easily show the spy. Note the similarity in outfit to the Twi-lek from the trailer, the use of cover, and the completely different look from the smuggler. All in all, I think the case for this class is closed. The evidence is stacked to the point where I can't see another alternative.