Well, off the top of my head:
The Three Religions of China (Taoism, Confucianism, Buddhism) were long considered to be the same religion or at least closely related. Most people believe in a syncretic version of these faiths, or at least, observe the culture of all three even if they have a specific path.
Tang Chinese authorities didn't seem to distinguish between Manichaens and Nestorian Christianity very much. In fact, in one place, the Manichean Religious Leader was ALSO in charge of the Nestorian Christian Community.
The Jesus Sutras of course were the stories of the New Testament, written in Chinese using Taoist and Buddhist terminologies and concepts.
Uh, the Early Chinese Buddhist Sutras were written in Chinese using Taoist terminology and concepts.
Islam and any "People of the Book" religions, which historically have included: Judaism, Christianity, Buddhism, Zoroastrianism, etc.
Islam is also compatible with Non-Nicene strands of Christianity, like the Monophysites (and these probably formed early converts to Islam in Eastern Roman territories).
I personally consider Buddhism and Islam to be very compatible, theologically speaking
— Infinite Buddhas. 124,000 Prophets.
— Periods of time with no Buddhas (Dark Kalpa). Periods of time with no Prophets (Ahl al-Fatrah).
— Devas are mortals, don't worship them. Djiins are mortals, don't worship them.
— King Mara is a deva who is the Devil. Iblis is a Djinn who is the Devil.
— The Maras are devas who follow King Mara. The Shaitans are Djinns who follow Iblis.
— Devas can have their own beliefs (they are not necessarily Buddhist per se). Djinns can be Jewish, Christian, Muslim or other.
And there are some Muslims who profess the belief that the Buddha is a Prophet of God.
The Buddha's stories of course eventually inspired the tales of two legendary Catholic Saint. Saint Barlaam, and Saint Josaphat.
The Early Roman Catholics did more or less just ripped off the Roman state religion wholesale (Jupiter = Jesus, and Juno = Mary).
And Catholicism and Buddhism are both very adept at adapting itself to local heterodox religions and cults. Catholicism integrates pagan elements, holidays and deities (sometimes rendering the deities as fairies or "legendary kings").
Buddhism just converts the pagan pantheons wholesale; as gods who have converted to Buddhism, and became "protectors of Buddhism". In fact the Buddhists did this with the Greek Pantheon as well. Which is why some Buddhist deities in Asia are actually Greek Gods, at least iconographically speaking. The most obvious example of this is Boreas/Wardo/Feng Bo/Fujin.
Hellenism and Mahayana theology, since Mahayana Buddhism developed in the shadow of the Hellenistic culture left by Alexander the Great. In fact, Buddhism was able to become a world religion thanks to the bridging of East and West achieved by Alexander (that, and Ashoka the Great's missionary efforts of course).
And philosophies. If we can consider Pythagoreanism, Platonism, etc as religions, they are pretty integrated with Christianity. Islam also draws from philosophy as well, and notably, Medieval Muslim scholars consider Pythagoreas (Hermes Trismegistus) to be the Prophet Idris. Philosophy can be key to understanding theology sometimes, for example, because the Buddha's discourses was all about countering Indian Logic to a certain extent, Chinese scholars didn't really get what the Buddhist Sutras was talking about, since China had no exposure to Indian Logic.
Any of the Abrahamic Faiths are easy to combine.
People have noted similarities in the Chinese God Shang-Di with the Abrahamic God, since the time of the Jesuits. Though Chinese Theology is Monistic, not Monotheistic.
In Nepal, Buddhism and Hinduism are the same thing as far as the locals are concerned. And Tibetan Buddhism seemed to have formed as a result of the merging of the local Tibetan indigenous shamanism with Buddhist theologies.
Mongolian Tengriism seems pretty compatible with Monistic and Monotheistic Theologies, and Genghis Khan was a huge supporter of religious diversity.
Ancient Egyptian religion, particularly it's conception of the Ma'at or divine justice, seems very compatible with the concept of the Greek Logo, Taoism, and Dharma.
Ancient Sumerian Religion seems fairly compatible with Abrahamic Religion. Or at least, the Old Testament (for obvious reasons).
Any Aryan religions (Vedic, Germanic, Greco-Roman, Celtic, Iranian) is likely compatible...because they were literally once the same religion.
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I usually categorize religions into two types: Ethnic and World. The former only really makes sense for a specific culture, or ethnicity. The other is actively trying to spread and is "universal" in theology. Ethnic religions arose first, and spread along with human migration. World religions really started with Buddhism, and has all of the hallmarks of what we think of in religion today: Missionaries, Universal Demographic, actively looking for converts, monastic practices or priesthoods, separated from ordinary geography or polity, etc.
