In the 5th Century BCE, there was a competitor to Buddhism and Jainism: Ajivika. Founded by a man named Makkhali Gosala, followers of Ajivika adopted a fatalist, deterministic outlook disbelieving in free will. They also believed in the concept of atoms, and they were atheists. However, they did believe in an afterlife and the concept of samsara like the Buddhists, but rather than being determined by one's free will, they believed everyone's path to nirvana was pre-determined by the cosmos.
Ajivika was fairly popular, and even received the support of the Mauryan Emperor Bindusara. However, Bindusara's successor Asoka was very pro-Buddhist, and he massacred more than 18,000 Ajivikas for their challenge to Buddhism. Asoka's dramatic promotion of Buddhism also helped popularize Ajivika's main competitor.
What if Asoka and later Mauryan emperors, rather than violently suppressing Ajivika, promoted that belief system instead of Buddhism, and it either became the dominant religion in India, and/or spread like Buddhism across Asia?