We only know a handful of things for sure about lifeline and they are:
1. There will be a new map.
2. Breakdown and campaign mode will not be compatible with lifelines. This is good news for anyone who decided not to buy Breakdown.
3. "A new way to play the game."
We can't do much more than speculate right now, but I think I found a hint to what's coming.
In the job board page for Undead Labs, there is a listing for experienced coders building economic infrastructure. This would be the equivalent of an auction house designer for an mmorpg. While this could very well be for the mmo that Undead Labs desperately wants to complete within the next decade, I'm wondering if maybe Lifeline will have some economic aspect to it.
I can argue against myself by stating that if it was meant to be in Lifeline then they would already have the personnel, since the new DLC is already being worked on.
Let's talk about Breakdown versus Lifeline. I think that Undead Labs is going to learn from what made Breakdown successful. First off, Breakdown doesn't end so don't expect Lifeline to have an ending either. Second, Breakdown does a good job of recycling content by randomizing your start location and all the other elements of survival. You can expect the same thing from Lifeline.
I think that the primary difference between Lifeline and Breakdown will be the home base setup. The word Lifeline makes me think of a supply line, similar to what you see in wars. I expect that Undead Labs wants you to conduct a salvaging war against the zombies. Instead of a home base you will have a sort of band of brothers who complete missions for npc towns and set up temporary locations to camp. Think of it as if you were able to move the camper around in Breakdown.
The location of npc towns and starting location could be randomized, creating a completely unique play through experience each time. Instead of always listening on the radio for Lilly's advice, you could be the leader of the pack giving it out.
This is what I want to see in Lifeline and I expect that building bases will vanish from this mode of the game. It'll all be about surviving on the go instead of turtlin' as one female survivor likes to call it.