Warriors Orochi/WMG

Masamune and Keiji being Orochi's underlings is a form of Take That to Sengoku Basara
Sengoku Basara 2 introduces Naoe Kanetsugu as a pathetic Joke Character a la Dan Hibiki. Koei doesn't like one of their main trio getting reduced like that, as a result, they snap back at Capcom by making two of Basara's poster boys (Yukimura is also Koei's poster boy, so he's out of question) underlings of Orochi that you need to beat down and label 'bad guys'. Not to mention Masamune, being the more 'main' poster boy of Basara, is turned into a worshipping fanatic of Orochi, stripping all his Badass points in his KOEI incarnation (it's less than the Capcom one, but STILL).

The third game will continue the story, and expand it to the next generation DW and SW (DW 7 and SW 3)
Which means, most maps get a revamp for nextgen maps, and they will include Bao Sanniang, Ma Dai, Liu Shan, Guan Suo, Cai Wenji, Jia Xu, Ding Feng, Lian Shi, Sima Shi, Sima Zhao, Wang Yuanji, Zhuge Dan, Guo Huai, Zhong Hui, Deng Ai and Xiahou Ba for the DW side; Kato Kiyomasa, Kaihime, Kuroda Kanbei, Hojo Ujiyasu, Tachibana Muneshige, Takenaka Hanbei, Mouri Motonari, Aya Gozen and Fukushima Masanori on the SW side. Plus Musashi and Kojiro returns.
 * Kanbei Kuroda, Will be one of Orochi's underlings.
 * Well Maybe Not, He is in the Samurai Team.
 * Fukushima Masanori will find a way to be Orochi's underlings. Because you know, KOEI hates him.
 * Jossed with both of them.
 * Sima Yi will bring the whole Jin faction to the Orochi side, but not everyone will be there for long. Zhong Hui would most likely stick with Sima Yi due to his arrogance. Sima Shi might also stay.
 * Jossed:
 * Sima Zhao, due to his carefree attitude and having Wang Yuanji might defect to another force. If Zhong Hui sticks, Deng Ai might defect. Xiahou Ba will definitely defect.
 * New Dream will have:
 * Aya Gozen, Fukushima Masanori and Gracia for the SW 3 XL.
 * Mitsunari Ishida, Kato Kiyomasa and Fukushima Masanori.
 * Kotaro Fuma, Hojo Ujiyasu and Kaihime for the Hojo Clan
 * Hojo Ujiyasu, Kenshin Uesugi and Shingen Takeda.
 * And Takamaru.
 * They might also be adding Warriors: Legends of Troy into the mix (assuming the game ever releases...).
 * And they have announced just such a Game! And not only they bring the DW7XL additions (Pang De, Guo Jia and Wang Yi), bring back Zuo Ci, Musashi, Kojiro and Goemon, they brought Achilles, they also brought in Jeanne D'Arc from Bladestorm and Ryu Hayabusa!
 * And Nemea from Z'ill Oll and of all people, Ayane from Dead or Alive.

The 4th game will have Kenshiro, from Ken's Rage
One hell of a pipe dream, but they don't call this section wild for nothing. Also, holy shit IT WOULD BE SO AWESOME
 * Koei is going to make a lot of anime-based Musou other than Gundam, Fist of the North Star and One Piece. THEN such crossover will be possible (Excepting Gundam because it'll be totally unfair for the ground characters who doesn't pilot Gundams)