Deadline is reporting that Sonequa Martin-Green has been cast in the lead role for Star Trek: Discovery. She will play Lieutenant Commander Rainsford, 1st officer aka "Number One" on the star ship and will be the primary perspective that show is told from. The actress may be familiar to fans as Sasha Williams on The Walking Dead. In theory she is still a cast member on the Walking Dead since that series shoots in the summer and fall while Discovery will film in the winter and spring but her casting strongly indicates her Walking Dead character doesn't survive the season. Star Trek has always been and ensemble with the captain usually being the lead character and perspective so the shift to 1st officer theoretically bring a new perspective.
Previous series first officers were Spock (TOS), Riker (TNG), Kira (DS9), Chakotay (VOY) and T'Pol (ENT). Not entirely sure how this will change the show's overall perspective. If think about it, with rare expections whatever the captain was dealing with, the 1st officer was essentially experiencing the same thing since usually literally standing side by side against whatever problem was the episode's focus. A large part of that is ultimately the role of 1st officer is to be the engine of story exposition. Characters were either explaining things to them (and thus the audience) or vice versa. At most I guess this just cuts out a middle man (the Captain). It will be interesting to see how this all ends up being executed.
Filming for Star Trek: Discovery starts soon for a May 2017 debut on CBS All Access streaming service.