There are two major problems that face an Elektra solo series. The first would be supporting characters. Because Elektra isn't one to form relationships, establishing decent supporting characters for her show will be difficult. This is an issue for The Punisher as well, but it does seem the Netflix writers have that one all figured out with Micro, Micro's family, Dinah, Karen, and others.
Elektra is an interesting character, and Elodie Yung is perfect in the part, but I don't think she has the ability to stand on her own yet. Not just because she hasn't been set up for it yet (no mission, no enemies) but because with particularly monstrous characters, especially a sociopath like Elektra who enjoys killing and the high she gets from it, a lack of a humanizing figure means that their story can be difficult to invest in. Also, Elektra is heavily tied to Daredevil, meaning that to get Elektra a standalone series, you would have to cut Matt out entirely to prevent it from becoming a Daredevil show. Problem is, when she's independent, Elektra shows some pretty disturbing and selfish tendencies without any particular redeeming qualities.