Based on the Daredevil Born Again trailer, it looks like the angle they’re taking is that both Matt and Fisk have given up being Daredevil and Kingpin, respectively. Matt says a line was crossed (maybe he thinks he accidentally got someone killed). Fisk may be trying to go legit because he wants to make amends with Maya Lopez. So Matt is just a lawyer now and Fisk is trying to be mayor.
But the two of them are forced to come out of retirement and join forces when Muse, the guy with the white mask, shows up and starts terrorizing NYC. And to emphasize just how bad Muse is, Matt has to also bring in Frank Castle to help. As for Fisk, he’s the one being targeted by Muse, so he has no choice but to work with his arch nemesis.
For the people who don’t know who Muse is, this guy is scary. I’m actually surprised Marvel is utilizing this character. He’s like Marvel’s version of Buffalo Bill or Hannibal Lecter.
Now, if we’re loosely following the comics storyline, maybe Muse is the villain for the first half of the show. Then, when he’s taken down and Fisk becomes Mayor, Fisk becomes the main villain since he starts implementing anti-vigilante laws specifically to target Daredevil and Punisher. Fisk also uses Muse’s rampage as justification that the city is in danger of vigilantes.