Elektra

The seed for Elektra's appearance on the series was planted in a flashback sequence in which Matt and Foggy reminisce about a Greek ex-girlfriend. That means that the character is definitely out there somewhere, probably being trained as an assassin, and that it won't be long until she finds her way to Hell's Kitchen. In the comics, Elektra was hired by the Kingpin to kill Daredevil. Based on what happened to Fisk in season 1, that storyline could easily play out in the show's sophomore year. Elektra is a monumentally important character in the Daredevil mythos and is due for some live-action redemption. Her appearance in the series could also pave the way for storylines featuring The Hand (which were also teased in season 1), so story-wise, it makes the most sense to bring her in sooner rather than later. Plus, we need more strong female characters in the MCU and few fit the bill as well as Elektra does.