If you find yourself running out of missions in The Division and your map starts looking rather barren, it might be the case that you need to unlock some more activities. To do this, head to any locked safe house and, after accessing it, talk to the JTF Officer and read the Situation Board inside. This will populate your map with new side missions and encounters, giving you plenty more activities to sink your teeth into.
Once you hit the player level cap of 30, this will also unlock Daily missions - these are harder versions of the main story missions and you get three of them to take on every 24 hours. Look for the missions markers which are no longer shaded out on your map to identify them, and you'll see confirmation that they are Daily missions if you hover over them. Complete these to earn new high end items and Phoenix Credits, which you'll need in order to purchase items from Advanced Weapons Vendors.