A war in the Persian gulf over the Abu Musa islands between Iran and the Gulf Cooperation Council countries.

Strait of Hormuz gets closed due to the hostilities. Oil prices go up, Syria goes haywire because the regime is already short on oil. Lebanon and Turkey get dragged into the worsened state of civil war inside Syria (power shift due to unavailability of tanks and other armed vehicles).

Iran responds by increasing 'revolutionary' works inside the Arab region, increasing support of Jihadi groups in the levant. Israel will bear the brunt of this arming of groups due to lack of control of their actions. Israel will start another Gaza or West Palestine war, maybe even Lebanon.

Depending on the president of Egypt at this time, the 1979 Egypt–Israel Peace Treaty might be revoked and Egypt might protect Arab states from Isreali aggression.

I mean, hypothetically speaking, but a war over those islands could cause some major instability in the most volatile and valuable region in the world.