Microwave onion, chili powder, oil, jalapeno, garlic, coriander, and cumin in bowl, stirring occasionally, until vegetables are softened, about 5 minutes; transfer to slow cooker.
Stir tomato sauce and sugar into slow cooker.
Season chicken with salt and pepper; add to slow cooker, and coat evenly with sauce mixture.
Cover and cook until chicken is tender, 4-6 hours on LOW.
Transfer chicken to a large bowl; let cool slightly, then shred into bite-size pieces; cover to keep warm.
Let braising liquid settle for 5 minutes, then remove fat from surface using large spoon.
Toss shredded chicken with 1 cup braising liquid; add more liquid as needed to keep meat moist and flavorful.
Stir in lime juice and season with salt and pepper to taste.
*Filling can be refrigerated in airtight container for up to 3 days or frozen for up to 1 month; if frozen, let filling thaw completely before using.