I'm going with a general post 2019 thread instead of going with a specific POD, like the Soleimani crisis going wrong, mainly because America going to war with Iran is difficult as there really isn't much of a demand for it in the country, nor was there ever. Yet, I do believe that if you can get something like the Iraqi Embassy siege leading to the deaths of actual Americans and a hostage crisis it could very well lead to such a demand gaining traction. Further, I stated "post 2019" mainly to get ISIS out of the way, though I do suppose that having its two biggest enemies fighting each other could lead to a resurgence on their part (further, not really plausible for America to go to war with Iran in a scenario where ISIS still holds territory and cities).
Iran has stated many times in the past that if a war occurs then they will use its various allies to trigger an all out regional war. This means the militias in Iraq, Hezbollah in Lebanon, the Houthis in Yemen, and could probably start some things with the overall Shia community in the Middle East/Central Asia. Further, this may also lead to a situation where the Assad government ends up getting entangled. Assad is, however, not stupid enough to go to war with America. Most likely scenario is that he openly turns his back on Iran (though Syrian intelligence might provide aid to the Iranian bloc) and makes Syria fully dependent on Russia. In doing so Russia would probably have to send in actual ground troops, thus leading to a possible scenario where Syria becomes another Afghanistan/Chechnya (but then, the rebels were already pretty badly defeated by that time, so they probably won't be as lucky).
These are just some brief thoughts. Iran could destroy the oil fields of Saudi Arabia, Kuwait, and the UAE, could also target the desalinisation plants of Kuwait, cause a big mess in Afghanistan by funding Shia groups which turn out to be just as anti-Taliban as they are anti-US, and let's not forget Israel-Palestine etc. As for those scenarios, I leave it to you to decide what does and does not happen.
Iran has stated many times in the past that if a war occurs then they will use its various allies to trigger an all out regional war. This means the militias in Iraq, Hezbollah in Lebanon, the Houthis in Yemen, and could probably start some things with the overall Shia community in the Middle East/Central Asia. Further, this may also lead to a situation where the Assad government ends up getting entangled. Assad is, however, not stupid enough to go to war with America. Most likely scenario is that he openly turns his back on Iran (though Syrian intelligence might provide aid to the Iranian bloc) and makes Syria fully dependent on Russia. In doing so Russia would probably have to send in actual ground troops, thus leading to a possible scenario where Syria becomes another Afghanistan/Chechnya (but then, the rebels were already pretty badly defeated by that time, so they probably won't be as lucky).
These are just some brief thoughts. Iran could destroy the oil fields of Saudi Arabia, Kuwait, and the UAE, could also target the desalinisation plants of Kuwait, cause a big mess in Afghanistan by funding Shia groups which turn out to be just as anti-Taliban as they are anti-US, and let's not forget Israel-Palestine etc. As for those scenarios, I leave it to you to decide what does and does not happen.