Honestly, who expected Nien Nunb? Who even is Nien Nunb? He's the "jowled, mouse-eyed native of Sullust" as Wikipedia puts it, who flew with Lando on the Millennium Falcon in the Battle of Endor in Return of the Jedi.
The reason the Star Wars Battlefront community suspects Nunb and Greedo is because they're shown in a teaser piece of artwork (below, via the Battlefront forum) for the Outer Rim paid DLC arriving March. The artwork shows a small battle outside Jabba's Palace with Nunb and Greedo front and centre. Why depict them, and no one else of note, if they're not included? Mind you, they could just be an artist's depiction of the Rodian and Sullustan heads already unlockable in the game, as pointed out in the comments below.
Greedo, in case you forgot, is the bounty hunter Han shot in the Mos Eisley cantina in A New Hope. It's what the whole 'who shot first?' debate is about.
All we know about the contents of Outer Rim so far are they'll feature a map based on Jabba's Palace and a map based on the factories of Sullust. Nunb being a native of Sullust further strengthens his cause, and Greedo being an employee of Jabba further strengthens his.
How will either of them play? Nunb may have support abilities a bit like Leia, and something to do with flying a spaceship although god knows what; and Greedo will presumably tread along the same lines as Han Solo and Boba Fett.
In total there are four new Heroes and Villains coming to Battlefront via paid DLC. Who else will be included? Given that they have to come from the era of the original films, it's not an easy question to guess at. Our bets are Chewbacca, Grand Moff Tarkin, Lando Calrissian, Old Obi-Wan Kenobi and... the villains are really tricky. IG-88?