I probably miss a lot of names, and focuses too much on Western Europe, I'm afraid.
LATE ROMAN EMPIRE & EARLY BYZANTINE EMPIRE
- Peter Brown : Focuses on the rise of Christianity and religion social roles.
- Averil Cameron : Specialist of Late Antiquity, especially Late Roman Empire and Early Byzantine Empire.
- Bertrand Lançon : Societal and political approches of Late Antiquity. (And author of "romains policiers*" using Late Antiquity background)
* Pun on "roman policier" (Detective novel) and "Romains"
WESTERN EUROPE
- Geneviève Bührer-Thierry : Focuses on Frankish (especially Carolingian) policies, society and its relationship with its peripheries (critically Germanic and Slavic)
- Roger Collins : Specialist of Visigothic Spain and its periphery (which makes him also interesting on Merovingian Gaul, and parts of North Africa)
-Marios Costambeys : A to-be-followed specialist on Early Medieval Italy, after the Lombard takeover.
- Clare Downham : Spcecialist on Viking Age in England and Ireland, and Irish Sea in Middle Ages.
- Bruno Dumézil : Focuses on the continuity of Romano-Barbarian kingdoms with Late Empire. Young historian, so others publications are expected.
- Patrick J. Geary : One of the finest references on Merovingian Gaul, and on the general aspect of Early Middle Ages
- Guy Halsall : Archeologist on migrations in Late Antiquity Europe. A must read, essentially as it goes against many misconceptions dominating outside academia.
- Richard Hodges : Especially for an economic understanding of Western Europe, and for a long-time consideration of the period, from a more archeological viewpoint. A proponent of the "Pirenne thesis", with several modifications, tough.
- Stuart Laycock : On Post-Roman Britain, and fairly interesting about it. He's not without faults, tough. (As politicizing-looking issues)
- Stéphane Lebecq : On Western European economy, especially in Gaul and Atlantic, but on a more historiographic lookout than archeologic, at the difference of Collins.
- Rosamond McKitterick : Specialized on Frankish/Carolingian written history and historiography. She's nevertheles doing a lot of others studies and vulgarization on other early medieval or medieval matters.
- Rory Naismith : Monetary history of Anglo-Saxon England.
- Michel Rouche : Mixes vulgarisation and precises studies on Early Medieval Gaul (Clovis, Aquitaine, relation with Church) but as well on societal matters (sexuality). His magna opus on Aquitaine is almost lost for masses (only 500 copies printed, and widespread in several private and public librairies. I've a partial copy, tough)
- Phillipe Sénac : Specialist of Early Islam, and especially its relations with Gaul and Francia
- Barbara Yorke : Specialist on Anglo-Saxon England, especially on societal matters.
CENTRAL & EASTERN EUROPE
- Michel Kazanski : Focuses on the archeology of Great Migrations, especially in Central Europe.
- Jiri Machacek : A name that you'll encounter a lot when searching on Great Moravia.
OTHERS
- Touraj Daryaee : Specialist on Sassanid Persia
(A late Antiquity continuing up to the IXth century is maybe a bit weird, so even if an arbitrary line have to be put, it may be interesting to consider VIIth century as a benchmarking period)