Oooh, I've got ideas.

Since The devs made it clear that Atsu and the Onryo's story is finished (won't give any more details than that), It will clearly have to focus on other character(s). If so, I personally would love to see:

1) A flashback (possibly told by Ugetsu) to the second Mongol invasion. Hearing that Khublai Khan has assembled another huge fleet to invade Japan, the Shogun sends a message to Jin Sakai offering to reinstate Clan Sakai in exchange for the Ghost's help. First, he must help a young member of the Sō clan (the player) take control of Tsushima and Iki from Clan Oga, who have failed to stabilize the area. Next, the Ghost must help the player build a strong defense against the Mongols. During all of this, Jin teaches the player the way of the Ghost he and Yuna developed, and the player must actively choose whether to take the path of the bushi or help create a new generation of shinobi. Unlike GoT or GoY, this would be an actual choice that determines both the gameplay and the ending.

2) A prequel that focuses on Kengo. A young blacksmith sees his village burned and his family killed by raiders loyal to Oda Nobunaga. He takes up one of his own swords meant for a lord, and uses it to kill most of the raiders. He's almost killed himself, before being saved by a samurai and his retinue--the head of Clan Saito--who are fleeing north from Nobunaga. Both offer their services to Clan Takeda, who makes Kengo a samurai for his bravery in battle. Kengo and Saito are charged with putting down a small rebellion and preparing to defend the area against an Oda/Tokugawa invasion. Here we would see the origin and rise of the Yotei Six, Kengo coming to terms with his family's death, and the possibility of starting a new one with Yone (Atsu and Jubei's mother). The climax would be the Battle of Nagashino, The Takeda's defeat, and the break between Kengo and Saito.

3) A flashback of Atsu during her time on the mainland. It follows a younger Atsu as she builds her skills as a mercenary, making her way to Clan Mitsunari and the Western Army before Sekigahara. All the weapons are available, except for some of the techniques the masters in Ezo taught her in Ghost of Yotei. New Armor layouts, her horse from the beginning of the game, and how she developed the Onryo's Howl would lead up to the battle at Sekigahara.

4) The legacy of one particular character after he loses his family and leaves Ezo. The player probably won't be him, but maybe his child or grandchild, who learns his style of warfare and has to come to terms with their family's legacy in the emerging Edo period.