Three major groups of Shiite, Sunni and Al-Qaeda (its affiliates) would clash with each other and cause a long period civil war in Yemen. In the long term, some members of Sunni group would undoubtedly join Al-Qaeda and its affiliates; exactly the same as what we have been experiencing in Iraq or Syria. To put it simply, the combination of extreme poverty, high rates of unemployment, weak undemocratic governmental institutions, the long and ongoing conflict between Shiite and Sunni groups caused by long period proxy war between Iran and Saudi Arabia and many other issues in the chaotic environment of Yemen would facilitate the best opportunity for the rise and emergence of new Islamic radical groups such as ISIS in this country. As such, it would be wise the United Nations would immediately find a peaceful and diplomatic solution for Yemen; before it's too late and the condition of this country has reached the point of no return.
This tragic event would be mostly devastating for Saudi Arabia in different terms. This event, consequently, would fuel the terrorist activities of Al-Shebab in Somalia. As such, we might predict that the Yemeni civil war would threaten the stability and security of some strategic areas such as the Gulf of Aden and the Strait of Bab-el-Mandeb somewhere in the future.