According to SiliconEra, a range of content drops are being released for Dynasty Warriors 9 in the coming months, among them an Additional Scenario Pack, which makes NPC officers Xiahou Ji, Dong Bai, Hua Xiong, and Yuan Shu playable. They'll get new scenarios, events, and more. This DLC will apparently be released in the summer, with other extra content set to include:
Hideaway Customization Pack
Release: April 2018
*The new Hideaway appearance items and furniture are acquired in the Trials.
Additional Weapon Pack
Release: May 2018
The site has gotten its information from Koei Tecmo, but that intel doesn't seem to be available in English-speaking countries just yet, so best to treat it as a rumour for now, despite the fact that it's very likely we'll get the content here too.