ISIS is a meance, but their power is exaggerated in the media. The Assad regime and the Iraqi government clearly have the upper hand in the civil wars in their countries.
If anything the rise of IS has been a gift to the Iranians. It gives them a perfect excuse to intervene more openly in Iraq and further them as a full Iranian client state.
The Islamic State has been an absolute Godsend to Iranian foreign policy.
The best case for Iran in such scenario would be creating a breakaway Shia state in the south of Iraq. There's no way they can control Sunni Arab and Kurdish areas.
Beside, attempting such a thing would not only risk a direct confrontation with the Saudis and other Gulf States, but also f**k up Iranian reapproachment with the West. Iranian are smart enough to stick with their current policies.
