None of the many pundits are asking what the "best" response by Iran (for their goals) would be
Everyone is speaking of what terrorism/military response Iran could opt for. However, if Iran's goal is to create a Shiite crescent led by Iran, would that kind of response get there?
My guess is that a major military or terrorism response could be the mirror image equivalent of the neocon inspired plan to invade Iraq and make it a thriving liberal democracy. Once started, no one really controls how it grows - all that would be predictable is that there would be immense bloodshed on many sides.
Much is made about the US/Iran conflict being asymmetrical and, of course, we are a far stronger country in every way. However, the same was true with Iraq, a smaller weaker country. However, there are some possible Iranian actions that could be better for them.
The Iraqi government was already angry that the US did not inform them on the attack the preceded the attack on the embassy. Here, the US killed the Iraqi Hezzbollah leader as well. Iraq is majority Shiite. One option for them is to demand the US and our coalition leave Iraq. Pundits have noted that they need us to protect against ISIS. However, this ignores that it was not just our coalition that fought ISIS. While true that Russia and Syria did little - preferring to fight the anti Assad rebels, whether they were with militant Sunni groups or not, it is undeniable that Iran helped push back ISIS. They did not coordinate with the US military, but they did act to insure they did not get in each other's way by "deconflicting". Imagine that Iran does not attack us, but backs Iraq in demanding the US leave, while they help Iraq defend itself from ISIS.
Then the US/coalition will not have a "friendly" country to invite them in in either Iraq or Syria. Using Turkey as a base is problematic because their goal is more to fight the Kurds than to fight ISIS. Consider that this sets up the situation that led to ISIS in the first place. In Iraq, the minority Sunnis had an increasingly small voice and the Sunnis were fighting Assad in Syria. Our ISIS plan was two fold, to push back ISIS from the land they controlled and to establish governance in the reclaimed areas to make the local Sunnis less likely to rejoin ISIS. If Iran replaces the US, getting to a peaceful place, will likely not be the goal. If their goal is to get a Shiite controlled arc from Iran, through Iraq, through Syria, this placing them in a more dominant position in the Sunni/Shiite conflict.
