1) Characters from SOD1 can be heard trying to talk to Lily, sounding like they're in trouble. If you hurry to the location of their broadcast (And I mean HURRY, like, you have 5 minutes to get there) you can recover the body of a Network agent that was the character's partner that didn't make it, who will have some nice gear on them. You'll have to fight through a large horde to get it, however, and if you leave the area before looting it, the body will disappear.
2) Sasquatch needs a favor. Using the helicopter model from Daybreak, Sasquatch will radio when you're away from base asking you to clear out an extra large horde from a specific location. He's got a sniper rifle, so he'll help you out as long as you don't leave the mission area. It'd be just kind of cool to see that helicopter hovering nearby with the added novelty of getting sniper cover.
3) Pastor Williams' treasure hunt. Literally just finding clues sprinkled throughout the map that point to a treasure trove of supplies left over from one of the old priest's efforts to help the needy during the outbreak.