I wonder, would a majority Hindu Indonesian Archipelago be considered part of the wider Indian region? Would it make Hinduism more prominent as a world religion and less tied to India in the minds of Westerners? Assuming limited butterflies, and that a) Euro-American history stays roughly the same until major contact, would there be greater interest on the part of Britain, France and Spain in the region, given that there'd be a major native power that might be swayed to favour them over their rivals? Or might the VoC kill the Majapahit Empire with a thousand cuts, like the British EIC killed the Mughals? The VoC, and then the Netherlands, might then elect to exploit the caste system to reinforce their rule.
Something unrelated, but still interesting, without wide adoption of Islam, the languages of the Archipelago (Bahasa Melayu, Javanese, Sundanese, Balinese, etc.) would probably look and sound more Indian. They'd lack the Arabic stratum that provides most of the words in the domains of religion and law, and would probably use an expanded Sanskrit/Pala stratum, much like Thai. They would also probably still use their older Indic scripts.
Architecturally, the islands would lack typically Islamic features like elaborate geometric designs, copious domes and minarets. Borobudur and Prambanan would probably not fall out of use, and so could have enduring influence on architecture. Wilwatikta might also survive to the modern day.
