And now since I apparently need to make this disclaimer: If you think you've seen the following ficlet before, you probably have. These were all posted to my tumblr weeks ago, and I'm just moving them. Please stop accusing me of plagiarising myself.
After Luffy had died, Sabo could barely get Ace to leave the small house he had shared with his lover. At first, Sabo thought it was just grief, just depression, just loneliness making Ace curl in on himself now that Luffy was six feet under, but since Sabo started spending more time in the house to help out, he had gained a different — crazier — theory.
Maybe it was the puzzle on the coffee table, which Ace would do a few pieces of before leaving it, and whenever he returned to it a few more would be done. Maybe it was the blanket, which seemed to hover over the couch instead of lie on it. Maybe it was the post it notes that would appear, seemingly new, in Luffy's handwriting. Whatever it was, it led Sabo's thoughts to ghost, which was impossible — but maybe Ace really believed Luffy's ghost was haunting his house; it could have explained his new shut-in behaviour.
So Sabo packed away the puzzle, smoothed down the blanket, tucked the post its out of sight. But it didn't matter. The puzzle reappeared in a different room, and the blanket seemed to push back, and the post its always found their way into Ace's hands somehow.
It did change one thing, though. Ace noticed what Sabo was doing, and he didn't like it. Sabo wasn't allowed to go near the puzzle, nor the blanket, anymore, and Ace started to hoard the post its far from Sabo's eyes. He became even more reclusive, confining himself to his and Luffy's bedroom rather than the whole house as he had before. He didn't let Sabo in, which was probably why Sabo didn't manage to catch on before he hung himself.
Sabo spent a long time considering whether packing the puzzle away again was worth it, but decided against it when he noticed that pieces kept getting filled in when he wasn't watching. He did move the blanket, though, and now the only blanket he was worried about was the one on Ace and Luffy's bed — it moved, sometimes, in a way that suggested something Sabo didn't want to think about. The post its came in Ace's handwriting now, too, and a lot of them were apologies and thanks. Sabo didn't know what to think.
Sabo might or might not have accidentally-on-purpose spilled a bag of flour in their kitchen, once, and sent the powder flying. He might or might not have gotten a glimpse of two human figures among the flour, holding hands. He might or might not have received a playful post it note in two different hands afterwards, teasing him for not believing.
He might or might not have kept living in Ace and Luffy's house. But, that was another story.