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 killed in front of him while trying to help her escape, and he likes Jaime's little sister Milagro quite a lot because she reminds him of his sister.
5: When Bart was in the future, he often wore black goggles after meeting Nathaniel because of his eyes. Whenever he was excited, worried, or scared, his eyes would shine yellow for a few seconds, then fade away. This still happens, but he can control it to being only about 0.2 seconds long, so only speedsters can catch it happening.
6: Bart is very skilled and athletic. He doesn't show it around the team, but he can actually dodge, flip, and block things very well, even without his speed.
7: Even though Bart doesn't show it much in his time, he actually would give his own life to save someone else's, because he just can't bear to see anyone else die in front of him.