The reasons why I believed Dabi was Toya are actually very unpopular- First off his eyes were a dead give away. If you look closely at all the Todoroki's eye color and eye shapes and then compare it to Dabi's eyes it's obvious. I also want to point out that Horikoshi did say he watched ATLA (Avatar the Last Airbender) and took Todoroki's character design from Zuko. That being the case, Zuko also had a older sibling with more powerful fire. This made me realize that Horikoshi was making a parallel with ATLA. Horikoshi deliberately paralleled Azula with Dabi, Todoroki with Zuko, and Endeavor with King Ozai. Also I just wanna point out that Dabi's scars make his un-burned skin look like a skeleton.. Maybe that's a foreshadow that Dabi will die in the future?