Riot Games' champion roadmap for League of Legends is going to get updated on 16 October 2019 but there is currently enough leaked info pointing to Lucian's wife, Senna, being the upcoming marksman. Riot's latest teaser all but confirmed it.
Senna was originally leaked in the form of unfinished splash art, shown above, and fans immediately started speculating it's her. She roughly resembles the woman seen in previous art and videos tied to Lucian and Thresh.
For those not up to speed, Senna is Lucian's wife. As League of Legends lore goes, the two of them were purifying the dark forces successfully until they came across Thresh on the Shadow Isles. The Chain Warden trapped Senna's soul, sparking Lucian's thirst for vengeance as he relentlessly pursued Thresh across Runeterra in years to come.
As you can see in the image above, the woman is carrying weapons similar to Lucian's, although it's not entirely clear where they came from. Each of them used to have a gun that looked almost identical to the other. Following Senna's apparent demise, Lucian took the other one, which is the reason why he is slinging light akimbo style.
After the hype died down, following silent months after the splash art's leak, Riot started announcing the 10 year anniversary plans and teasing a new champion through in-game effects. After that, one Rioter confirmed the champion roadmap would get three new entries on 16 October 2019. Shortly after that statement, Riot Games released the teaser you can see below.
Come 16th, we should see what the former purifier will look like as there is barely any doubt she will appear in a corrupted form, possibly serving the Shadow Isles.