Okay so my theory is that Elena and Damon both died at the same time, like in The Notebook. They’re walking into this peace world together, and then I think Elena finds peace, which is why Damon disappeared, and very soon after (maybe at the same time again) Damon finds peace. When Damon found peace, he found himself at the Salvatore boarding house to see Stefan. I think that it’s all connected. I like to believe that they all get to see each other.
Elena and Damon don’t separate even in the afterlife. I imagine that Damon, Stefan, Lexi, Elena, Miranda, Jenna, Grayson, and John all sit together and talk about their lives and adventures. At first I think Grayson might be hesitant about Damon, Stefan, and Lexi considering they were vampires, but after learning that Elena became a vampire, and their adventures and how the Salvatore brothers helped her, he becomes more open and inviting to his son-in-law and his brother and his brother’s best friend. Even John who didn’t like Damon very much warms up to him.
I think they sit around, eat, tell stories, and do everything that should’ve happened if Grayson, Miranda, John, Jenna, Stefan, and Lexi didn’t die. Just be a family. I like to think that Damon and Elena tells stories about their kids. They had 2 kids: one boy, and one girl. They named them Stefan and Miranda.