I have a couple of suggestions for more realistic Roman provinces, I hope a couple of them will be implemented in the 1.0 version of the mod
-Pannonia should have the two counties South of the Danube which are currently de jure part of Dacia since the Roman limes was located at the Danube and a resurgent Roman empire would have surely reclaimed them. A new duchy called "Pannonia Valeria" could be created to facilitate that. Also I noticed that there's two counties in the game named Gorsium within Pannonia, so renaming the Northern one into Aquincum would make sense imo, since it was a large Roman settlement in the area.
-Moesia inferior should not be able to be conquered via the Roman reconquest CB, since it leads to the ERE expanding further North than it naturally would. Instead the parts of the kingdom South of the Danube could be given to the kingdom of Thrace, and the remaining parts made into a new kingdom not connected to the Roman empire (possibly Sklaveni?). The name Moesia Inferior could still be used as a duchy level title within the kingdom of Thrace.
-Taurica should not be a de jure part of the ERE, since it also leads to the ERE expanding too far into the steppes. The Southern half of the Crimean peninsula (Cherson) could potentially be de jure part of Pontus, or the entire kingdom could be a de jure part of the Empire of Scythia, I'm not too sure which would be better in this case.
-Colchis should be a de jure part of the Empire of Armenia instead of the ERE, since it was never an integral long term part of the Roman empire, and Roman control in the area was tenous at the best of times. Also it inevitably leads to the ERE swallowing up potentially interesting Georgian Zoroatrian and Armazi starts, which could easily be avoided.
-Pontus should be expanded Southwards and include the kingdom of Galatia and the Westernmost duchy of Armenia (I sadly forgot the exact name, I think Coloneia?) as de jure parts, since that was the state of the province in late Antiquity. Also it would make the borders a lot smoother in the area.
-Oriens should include the duchy of Arabia Magna as a de jure part, both to make the borders smoother, more historically accurate and prevent Sassanid de jure claim attacks over traditionally Roman territory. Possibly the kingdom of Cilicia could be integrated into Oriens as well, since it historically was part of it, but it might make Oriens too powerful, so I'm unsure if it should be done.
-Achaea should be integrated into Macedonia, since it was part of Macedonia during late Antiquity. Also the island of Crete could be added to Macedonia for the same reason.
-Egypt's de jure borders should be expanded Southwards to include the cities of Syrene and Berenice, which were historically part of Roman Egypt.
-Tripolitania should be split into new, smaller counties along the Mediterranean coastline (in the same style as the two current Vandal counties in Tripolitania) and large desert counties in the hinterland. The kingdom should be devided between Africa who will be getting the coastline and Garamantia, who will be getting the hinterland. This would reflect historical control of the coastline better. The coastline could be made into a new duchy named Tripolitania.
-The kingdoms of Brittany and Burgundy should be made de jure parts of Lugdunensis, since they both contain only one duchy and are also historically tied to the region.
-The last one may be rather small, but would nonetheless make the mod better: The border of England could be pushed slightly further South to the historical location of Hadrian's wall along the river Tyne. The land to the North could be made into a new duchy of Caledonia, so players who wish to give England its modern borders can do so.
I hope you like my suggestions and of course I'd love to see some of them in the mod some day! Keep up the good work, definitely the best CK2 mod out there.