Since there's quite a lot of predictions from you guys, I'd like to discuss about mine (just personal opinion):
After fights, Yona finally gets to Kouka kingdom. Soo-won wants to reconnect the broken relationship but she refuses. She states that she can't be friend with the one who killed her father and tried to kill her. Although Yona does not forgive him, she wants Soo-won to be a good king and rule the kingdom till the end of his day. Yona, Hak and the dragons move to Wind tribe and live there, Yoon comes back to his home with Ik-su. Kija, Shin-ah and Jae-ha become immortal as they have found the red-haired king, but cannot have their bodies renew like Zeno; they can only stay young forever. Yona ends up with Hak and marry him maybe <?> Soo won may end up with Lili; Tae-jun continues to help the needy. Yona tells the four dragons to continue their journey together; find the "king" when she passes away.
Well that's the best ending i can think of since I love happy endings. Okay bye guys have a good day