Challenge: Make Buddhism, Zoroastrianism, Jainism, and Nestorian Christianity most popular world religions.
Making Zoroastrianism, Nestorian Christianity and Buddhism major religions isn't too hard; butterflying Islam away and Zoroastrianism and Nestorianism are bound to remain major religions, and Buddhism already managed to remain a major religion IOTL.
Making Jainism a major religion will be rather tricky, though...
...and making sure that these religions eclipse Hinduism and the non-Nestorian forms of Christianity will be pretty hard...
If the Khazars, Volga Bulgarians, or another Turkic people in eastern Europe becomes Nestorian, then it shouldn't be
too hard to get a Nestorian Russia - but that's about as far as Nestorianism could realistically expand into Europe, even in a best-case scenario.