4) Mister Fear
After Kingpin, Bullseye and the Owl, Mister Fear is probably the next most significant Daredevil villain. When your hero is known as the Man Without Fear, it is irresistible that he would come up against someone who challenges that by using fear as his weapon.
Yes, Mister Fear is rather like the more famous Batman villain Scarecrow, as both utilize fear gas, but using similar characters has never bothered comic book adaptations before (Arrow/Hawkeye, for instance) and Mister Fear could be used very differently from that other villain.
An interesting Mister Fear story to adapt could be the comics storyline ‘Without Fear’ in which he used a drug to remove the fear of death from everyone in Hell’s Kitchen, making the population near-psychotic and a danger to themselves. Matt Murdock would definitely have his work cut out if this plotline was used in the next season of the show.