Iran transitions to a modern, democratic country. It would be a lot more like Turkey, which until the last few years was doing very well for decades. Good relations with the west become possible (despite the short term tension caused by oil nationalisation) and the Iran-Iraq war never happens.
That leaves Iran much richer than it is today, because without the immense damage inflicted by the war and its aftermath combined with the country's bad relations with the world has really caused Iran to suffer. Remove those factors and the country could really be a bright spot in the whole region.
Even as it is, Iran is arguably the strongest Muslim country today, with the possible exception of Turkey.
Without the Iraq Iran war, Saddam likely never invades Kuwait, which means no Gulf war either. That in turn means probably no western invasion in 2003, which means no ISIS. So Iraq would be in a far better place too.
Being able to trade with the world and not having your physics professors assassinated by the Mossad would certainly help. Without the instability of the revolution the Iranian diaspora would be much smaller. Tehrangeles, the Iranian neighborhood of LA, might not exist.
Decades of a stable, secular Iran would also provide another model for the new republics in Central Asia. OTL, Azerbaijan and the new Central Asian republics built their states in Turkey's image, to the point of copying the Turkish Constitution almost word for word in some cases.
A secular, democratic Iran would provide a different country to look to, and Pan-Turkic sentiment would have a more significant competitor. Even today, Iranians view as Azerbaijan, Tajikistan, and Afghanistan as a cultural "greater Iran" with the Kurds and other central Asians sometimes thrown in.
Tajik isn't really a separate language per se, it's just a dialect of Persian that was twisted the same way Moscow also tried to meme a "Moldovan" language into being among the Romanian speaking subjects they ruled.