1: Bart has a bunch of scars from being a slave. Arms, legs, back, basically everywhere, which is why he only wears jeans and long sleeved shirts.
2: Bart has nightmares of the future every night. He often wakes up at night sweating, sometimes screaming, and on the verge of crying.
3: Bart had siblings (I like the sound of 2 or 3), but they either died from sickness, exhaustion, or were killed by Blue Beetle. Thad Thawne being his older step-brother, and he doesn't know what happened to him, but hasn't seen him since he was 9 or 10.
4: (This one relates to Headcannon #3) Bart's little sister was...