Dead or Alive's Marie Rose and Honoka are just not satisfied with being in their own game. Originally DLC fighters in Dead or Alive 5, they were titular characters in Dead or Alive Xtreme 3. They have since continued to amass popularity and thus have began invading other games, starting with Musou Stars. This time they will be playable in Senran Kagura: Peach Beach Splash via DLC.
They will be available on March 30, 2017 in Japan, where the main game releases March 16. Peach Beach Splash is coming west, even if we don't have an official release date yet (patiently waiting). In the meantime we have a video of the first 35 minutes to tide us over.