The Roadmap notes that the champion will come from an unknown faction, a group deep in the jungle. The image associated looks like some Hextech-using civilizations like Piltover might be trying to tear down the forests for resources, but some people aren’t really feeling that.
This is all reminiscent of Nidalee’s old lore, where she was a girl raised in the Kumungu Jungle, who also protected said jungle from war and nations trying to cut down the trees.
The game’s last released AD assassin was Pyke in May 2018 and Kayn in June 2017. Sylas technically is classified as a skirmisher, but he’s not AD.
Another champion was teased, but extremely lightly. All we know is that they’re going to be a non-traditional marksman, and they probably use energy or light, just as shown in the animation included in the Roadmap.