1.) The Zanj rebellion and the fall of the Abbasid period. Other than my tl I find this to be one of the least explored areas of history on this tl compared to how important the turn of events were. The tl I have is still ongoing, but time constraints are stopping me from moving at a rapid pace. Regardless, the era is extremely important to world and Islamic history, yet is barely spoken on, within this site except by me.
Within the above subject is:
Zanj Rebellion, most bloody war in Iraq's history since the fall of Assyria. The rape of Basra and depopulation of the most fertile land in Iraq led to a greatly weakened Iraq by the 1200s.
Anarchy of Samarra, the power struggle of the Abbasid court between soldiers of factions. Quite a bit of political intrigue.
The rise of Ya'qub bin Laith al-Saffarid, the first "free" Iran in a sense and culminated in the great Abbasid and Zanj victory against the upstart Saffarids.
The rise of the Saljuks, and the physical representation of Turkish dominance.
The Qarmatian invasion, the destruction of the Kaaba and the rise of a millenial Shi'i cult.
The Khawarij revolt of Ninewah, the depopulation of Hulwan and the battle of Mosul, as Arab Khawarij attempted to wrest power from the Mu'tazilite Abbasids, all over a police murder (interesting in light of today, huh).
The short lived independent Tulunid dynasty of Egypt, who knows what they could do.
The powerful Buyids of Iran. Still another example of limitless potential.
The state of Batihah, the anarchic home of criminals and thugs. Holding out within the swamp, a pirate like kingdom of fugitives, many veterans from the Zanj wars, created a powerful and feared state in the swamps of Southern Iraq.
Etc, the list goes on.
In terms of others:
A Khazar movement continues its gains during the zenith of the Umayyad-Khazar wars and forms an alliance with Byzantium, perhaps Byzantium takes back both Anatolia and Syria and the Khazars capture land from Georgia to Mosul, Abbasid-Khazar-Byzantine war?
A scenario I have always wanted was a Gupta-Umayyad war, it would be climatic and would be interesting to see in what ways the Umayyad adjust to such a colossal for. Perhaps, a serious Byzantine-Umayyad truce or Proto alliance?
A Umayyad victory in the Khazar-Umayyad wars. This could postpone their fall for decades and interestingly, I say, give the Byzantines a better chance.
I have never seen an Umayyad tl. Perhaps I will do one at some point.
Al-Andalus is the most covered and saturated Islamic subject ever, stay away from it!
A tl on the Hashashin or other such Shi'i movements.
The Saljuk-Fatimid conflict is rarely covered.
The extremely under covered early Islamic period of the Murji'ah, Shi'i and Khawarij. Imagine a tl starting with the arrival of the preacher, Ibn Sabah, during the reign of Uthman and going into the conflict between Ali and Muawiyah then in to the battle of Jami'jan between the caliphate and an alliance of Murji'ah, Shi'i and Khawarij revolutionary.
All of these are serious heavy hitting Islamic topics of extreme importance, thus I do not feel it is unfeasible. I however would take simply more discussion on these topics, as likely no one, except myself would be willing to write on most of these (I could not write a tl on the Gupta without assistance and co-planning with a poster with considerable Indian history knowledge).