Remember, these are my personal headcanons, and aren't actual canon (or at least not confirmed)!

Dream is thousands of years old

Dream had a crush on Techno when they were in their teens, but kept getting rejected so he started chasing after George

Dream is a shapeshifter

Dream has a blob pet that transforms in his mask (so when he's not wearing his mask, the blob is just vibing)

Dream can control Ranboo in his enderwalk state

Dream can make Ranboo go into his enderwalk state manually

Dream had a mistress during an era where he traveled the lands, and ended up knocking her up and she ended up having a kid (in other words, Dream has a kid)

Dream is from a dimension that is no longer accessible and has been long forgotten by mankind (hint: this is a reference to a very old Minecraft thing that was removed!)