According to Respawn Entertainment community manager Abbie Heppe, Titanfall's upcoming DLC content could include some creatures and monsters that may allow players to discover the various alien species within the lore of the game.
Despite three DLC packs in the works, it seems as if Titanfall aren't yet 100% sure what the three packs will consist of, but we all know new maps will be included. When asked about aliens and monsters, Heppe stated that, "We are going to have maps added, and we're trying to give players as much of a varied look at things as we can so that is definitely a possibility."
Many maps in Titanfall already have various creatures and spaceships in the background that can be killed for extra experience so it's possible that the DLC maps will make similar creatures more of a focus, instead of just a background element.
Whatever content the DLC packs include, Respawn seem to be keen on allowing players to explore the world of Titanfall even further, and it could be possible that these DLC packs will included more scripted content, similar to the current campaign offering available already.
As for other content, Titanfall DLC will most likely not include any new Titans, as Heppe has admitted that it's a huge undertaking to integrate a new Titan that is both a useful asset to the game and balanced enough to not be either underpowered or overpowered.
It could be possible that we see more weapons or pilot equipment though.