This article, from a pro-Saudi source, explores the ramifications for Iran if the US designates the entirety of the Islamic Revolutionary Guards Corp (IRGC) as a terrorist organisation. While the article only focuses on the positive outcomes (for the West and Saudi Arabia), and entirely minimises the role the Saudis have played in cultivating Sunni Islamists across the region (which has culminated in Islamic State), it’s still worth the read.
I note that Australian law makes clear that any organisation that supports terrorist activity may be designated a terrorist group. The IRGC does that, and more.
But the negative ramifications of designating the IRGC should be kept in mind. Iran would seek to punish the US and its friends; cue attacks against Saudi Arabia and Israel, and on Jewish targets internationally, by Iranian proxies such as Hezbollah and Islamic Jihad.
Iran would also use the move as an excuse to abrogate the nuclear deal, and quickly accelerate its march to nuclear capacity and weaponry. (Beyond a desire to export its Islamic Revolution and influence across the region, Iran’s rulers are also deeply suspicious and paranoid about Western designs on Iran; the nuclear option is seen as an excellent defence against such nefariousness.*)
* This has long been used as an argument by apologists for Iran that the West ought to cut down its aggression against the Islamic Republic. But no matter what Western policies are or have been, Iran since 1979 has seen them as being designed to attack Iran, so it makes no difference.