I'm calling for double suicide ending, it's been a while reading manga with that kind ending. Judging by the story development I think it's quite possible.
Kita is the man who thought that his life pretty much over and and want to go wild for the last time, Yuki is typical neglected high class wife with slow boring life (quite possible with cronical disease like cancer).
In the next development, I can't wait the author write an arc about Yuki's reason why she wanted coming along with Kita (my wild guess will be, Yuki and her husband were so madly in love back then -behind the reason why her engagment ring special for her-, but now like typical ntr/cheating hitozuma manga her husband became so busy, and her illness made it (what yuki feels about their marriage) worse -and I think she's hiding her illness from her husband-