The reasons Dabi is Shoto's eldest brother Toya since his origins are unknown. Also Dabi knows the game Shogi and hates fish.
2. The canon name of Dabi's quirk is still unknown but his blue flames are stronger and hotter and a flashback in Chapter 202 reveals that Toya's Fire Quirk is stronger than Endeavor's Hellflame and like Izuku with One for All both quirks are too much for their bodies to handle which results in purple colored skin it could even happen to Shoto (even with his ice side) or Endeavor since blue fire is stronger than red-orange fire.
4. In Chapter 191, Dabi admits that he could never stopped thinking about Snatch's words implying that he empathizes with what it feels like to have a broken family and in Chapter 192, Natsuo mentions Endeavor was responsible for an unknown incident with Toya.
5. Dabi didn't commit any crimes before joining, he didn't want to tell his real name to Tomura and to add more of his distance from the League/PLF is 1) he's the only Vanguard Action Squad member who's real name wasn't revealed during the Hideout Raid Arc and 2) unlike Tenko, Jin, Himiko and Shuichi, his origin's aren't revealed during the Meta Liberation Army Arc. Also in Chapter 191, Hawks is left in the shadows of darkness and Dabi is walking toward the shining doorway of light similar to All Might in Chapter 1/Episode 1.
Does this mean that the next arc or the next will focus on Dabi (if he's Toya Todoroki) and possibly the Todoroki family?
6. Dabi's eyes are full turquoise like Enji's, his face resembles young Enji and also Shoto takes after Toya/Dabi (if they're the same person) physically and in more ways than one (excluding hair and eyes expect the left side of Shoto).
- Seen between the way they activate their quirks:
- Their eyes: Dabi's eye and Shoto's eye
- Neck rubbing: Dabi and Shoto
- Similar expressions: Shoto and Dabi, Shoto 2 and Dabi 2 and 
- Similar builds.
- Both are kicked in the head, it happened to Shoto in Chap.41/Ep.24 and Dabi in Chap.87/Ep.47.
- They both do this steam/smoke and mist venting.
- Both use ranged combat with they're quirks, which can be erased by Shota Aizawa and they get caught by his binding cloth.