If Islam was never created, do you think Zorastrianism (which mainly existed in Iran but also in other areas-and still exists to this day in isolated areas like India), would have become a member of the "Big 3" monotheistic faiths alongside Judaism and Christianity?
Zoroastrianism didn't have the missionary impulse that Christianity and Islam did. I wouldn't call pre-Islamic Zoroastrianism "monotheistic" in the same way either. Mithra, Anahita, Verethragna, and company are called "yazatas", meaning "worthy of worship". They were often equated with the Greek gods during the Seleucid and Parthian eras within Persian territory proper, indicating that they were seen as something more than the angelic interpretation that arose due to pressure to seem like a "People of the Book" religion to Muslim caliphates. All my reading on the topic suggests to me that ancient Zoroastrianism was polytheistic.
("Magians", aka Zoroastrians, are mentioned in the Qu'ran, but its stance on them seems unclear to me other than 22:17, where they will be judged by Allah along with Jews, Christians, and "Sabians" according to Muslim eschatology.)
During the Sassanid era, you could argue that Zurvanism was the main branch promoted by the state. One of Shapur II's daughters was named Zurvandukht after the god of time above Ahura Mazda in the Zurvanist sect, for one anecdotal example.
The problem with studying Zoroastrianism is that they either didn't write down their records until the Islamic period, or they had exceptionally bad luck in losing them. Zoroaster himself is rarely mentioned, and his date of birth has been placed anywhere from the 1500s BC to the 550s BC, depending on whom you ask. (I think the religion started around the earlier dates, due to the Avestan language's close resemblance to the Rig-Vedic language)
If you want to know more about Zoroastrianism, check out these authors:
Ehsan Yarshater (general reference works about Persian history)
Mary Boyce (books about Zoroastrianism)
Amelie Kuhrt (more so for Achaemenid period in general)
William Malandra (a book about Zoroastrianism)
A Zoroastrian site that can give some introduction to Zoroastrian topics and scriptures is Avesta.org
(I wrote a thesis years about Zoroastrian history up until the Parsi exodus to India, in case you're wondering why a guy calling himself "Herman Gigglethorpe" is talking about this topic. . .)
EDIT: I should mention that Zoroastrianism declined only after the Muslim conquest of Persia, and even then it took a few centuries. Muslim rulers complained about having to bribe Persians to go to Friday prayers! A timeline without Islam means a lot more Zoroastrians.