I dont think throwing some weapons and missions would work.
They could probably make something like the GTA4 DLC only smaller scale - take some important NPC from the main story and make him/her the main character of the DLC (did someone require female PC?). He/she could use different weapons/equipment to make the gameplay feel somewhat different (sniper rifle?). The story could be somehow connected with the main story - let us see the main story from different perspective. Maybe even some decisions from the main story could affect the story of the DLC in some way (or maybe not). The dialogues should reflect the fact the main character of the DLC is not Michael Thorton - both main character dialogue options and NPC reactions should feel different (no JBs).
I like the Laras Shadow DLC in Tomb Raider: Underworld - it was connected with the main story (and with the other DLC), but playing as the Doppelganger felt really different as the DLC introduced new gameplay mechanisms and changed/removed some existing.
Now, why am I even writing this? If there is going to be DLC, they are working on it by now. Writers, artist, designer are probably finished with this game and unless they have moved to another positions (QA?), they have been working on other things for months.