Much of southeast Asia was influenced by itself. Indicated by the trade routes on the map below, it shows how the different empires/kingdoms of Southeast Asia traded with each other, which means that their different cultures would be blended slightly. The trade routes also show how those empires traded with India and China, which would mean that Indian and Chinese cultures also influenced the cultures of Southeast Asia.
Southeast Asia contributed A LOT to the spreading of different cultures. They were responsible for much of the spread of certain major religions, such as Buddhism, Hinduism, and even Islam.
One of the major kings of the Pagan Empire was known as Anawrahta (ruled from 1044 to 1077). He is the one responsible for much of the expansion of the Pagan Empire, and he gained access to trading ports due to his conquests, which allowed his empire to prosper. He also encouraged and increased the spread in Buddhism, since he was a supporter of Theravada Buddhism himself.
Jayavarman II was considered to be the founder of the Khmer Empire, and also was known as one of the greatest leaders of that area. He was responsible for the deification of the Cambodian monarchy, the establishment of the official state religion, and the unification and expansion of the Khmer Empire.
Other rulers at this time were responsible for extensive and glorious temples being built, one of which being the Angkor Wat, which is still standing today.