In terms of historical subjects, my main areas are as follows:
Abbasid period both in terms of military history, politics, intrigue, culture, science, religion and law codes. Events within that especially interest me within are; Zanj revolt, Saffarid-Abbasid war, Ya'qub ibn Layth al Saffarid, The effects of millennial revoltism in both Shi'i and Khawarij Islam which then can be extrapolated to the overall state of the Abbasid, Turkish-Arab cultural interactions, etc, etc, etc....
Umayyad period especially in terms of military history and development of the 'new Arab.' The wars between the Ummah and Byzantium, Khazareim, etc and then the Berber revolts and the Abbasid revolution and the war with Abu Sayj all are interesting to me.
Shi'i Islam, Khawarijite Islam and other similar movements (Murji'ah) influence on Islamic history and application to contemporary Islamic fiqh and rulings.
Hero worship in the period's of early Islam. Such as the virtual worship of Yahya ibn Umar and of the Imamah of the Shi'i or the later worship of Fatimid rulers. This trend also extends to perhaps its most interesting case, being Is'mail of the Safavids.
Islamic scholars and faqih in history and their influence of statesmanship in the Islamic world.
Mamluk sultanate of Cairo, in most ways. Both in terms of culture and military history.
Iran from the Parthian period to the Safavid. Especially, Iran from the Islamic conquest to the Safavid empire. Including Iranian-Arab cultural transfers.
Saudi history pre 1900.
Many others of course.
Lesser known areas that I know well enough information and ideas on.
France in the 16-1700s and colonial Louisiana and Canada.
Language in modern and near past Louisiana and United States.
Bronze Age Middle East and Bronze Age/Iron Age Arabia.
Neo-Assyrian Empire
Christian religion and development through the ages and its relation to Islam. Such as drawing conclusions of similar ideas and influences.
Manichaeism and it's relation to Islam in the Abbasid period.
Anything European or Byzantine related. From Rome till the Renaissance, not my strongest area however.
Areas that I am especially weak in:
American history
Napoleon and related subjects
Chinese history outside of its relation to Islamic states
Japanese history
Dutch/Germanic history
Europe in the dark ages outside of transfer with Islam.
Southeast Asian history
History of imperialism, such as colonization of Africa or Australia.