I really enjoyed this one:

https://www.fanfiction.net/s/6046340/1/Coming-Down-on-a-Sunny-Day


In 2004, Castiel watched the Apocalypse end disastrously and makes a decision. In 1984, John Winchester suddenly finds himself with another little boy, one with dark brown hair and sad, blue eyes who won't leave John and his sons alone. And so Sam and Dean get a weirdo big brother. Because both of their lives aren't complicated enough.

Time travels obviously, really cool story. Hope it helps ya!

Robyn