Answer:
The major ideas were religion i.e. Hinduism/Buddhism and the Culture and language.
Step-by-step explanation:
The Brahamins of India spread their religion wherever they went in South East Asia. The idea of an after life and a caste system became extremely popular with Royalty and in many places it became the state religion.
For poorer folks the idea of Bhuddism and the belief that everyone is equal was a major attraction.
As two regions became close due to religion, there was a natural indianization with key cultural symbols and even the Sanskrit language travelling to far flung places in South East Asia.