So some things about DLC.

They COULD, release the some of the preorder bonuses out for free (wise choice) or with a price for everyone.
They COULD, just release another map via a part of the Ark they didn't put in the retail game from the actual universal map.
They CAN, make new challenges for the exp gain, or maybe to reward people with my next idea below.
They CAN, just throw in some new Archetypes or just pieces of armor that isn't an Archetype as a sort of pack via Security or Resistance.

Any idea of NEW weapons or attachments, is silly. This game is about balance. Not everyone can buy or afford DLC. And if these weapons are powerful or easy to get then most of the powerful guns in the game, not quite fare.