Buddhism dominating India is not too difficult to conceive, because it actually happened in history. In fact when Ashoka adopted Buddhism, it might have become the majority religion in Mauryan Empire which covered almost the whole of the subcontinent and beyond. Of course, the Brahmins resented their loss of influence and one of the reasons for the fall of the Mauryan Empire was the Brahmin revolt. The Mauryan Empire established by one brahmin, Chanakya was terminated by another brahmin, Pushyamitra Sunga, who was the commander-in-chief of the Mauryan Imperial Army. Ashoka, though he propagated Buddhism did not suppress or discourage Hinduism. Many emperors like Harsha Vardhan encouraged both religions. In fact Hinduism in its modern form did not exist at that time. Vaishnavism and Shaivism were rival faiths who considered Vishnu and Shiva as their supreme deities. Both sects while fighting Buddhism and Jainism also fought each other.
Later Vaishnavism and Shaivism joined together making Vishnu and Shiva as equal members of the Trinity. Brahma was added as the third member but with less power. Other sects like the Shaktheyas also joined. Buddha was proclaimed as an avatar (incarnation) of Vishnu and many practices of Buddhism were adopted into Hinduism. The Gupta Empire formed in the fourth century promoted Hinduism. The Gupta period, known as the Golden Age of Ancient India also saw the decline of Buddhism.
Later in the nineth century, it was Shankaracharya who adopted many Buddhist principles and propagated Adwaita philosophy who re-established the supremacy of Hinduism. For this, he borrowed Adwaita, which was originally devised by Buddha and established it as the foremost philosophical principle of Hinduism. He adopted the Sangha structure of Buddhism and established four monasteries at the four corners of India, Badrinath in the north, Puri in the east, Sringeri in the south, Dwarka in the west. In short by adopting the philosophy of Buddhism and the organisation of Buddhism, he banished Buddhism from its land of birth. That's why Shankaracharya is called "Prachchanna Buddha" (Buddha in disguise).
If Ashoka, instead of being tolerant, had made Buddhism the sole state religion and banished other faiths, India might have remained Buddhist for future. If a Buddhist Mauryan Empire had lasted for a few centuries, India too would have become Buddhist like SriLanka, Thailand or Burma.
