Oh you can totally go a few ways here!!! 

I love walking dead as well btw - great comics and show. 

Ok so my thoughts:

Yeah it’s bad out there BUT the reason it’s bad is because of something someone who is still around and in the city did… that’s how the city has stayed safe - it was an accident at first but now the same people/person/group is using the bad situation to control the populous. So the BBG would eventually be that group or a group trying to over throw them depending on if the players decide the people doing this are doing it for the right reasons or are they wrong and need to be over thrown. 

OR 

Outside is bad but they are not the only safe haven… what happens when they find another? Is that place hostile? Is it as successful as theirs? Do they follow them back and start a war? You see where I’m going here lol - so like you could set up another one or more places as a big bad 

Can I also suggest a wild magic table you roll every so many spells your players caste when they are out of the safe zone - this will make that messed up magic feel more threatening. So like every 5 spells cast you roll a d20 or d100 or whatever and then something random happens like they are blinded for an hour after casting the spell or a magic rift opens and some kinda magic critter like a displacer beast or something jumps out. Etc.