This.
Soverihn has hit the nail right on its head. Of course this is escapism- kind of the whole point here- but if this goes isekai, it’ll become a joke; it doesn’t need to be isekai, and frankly speaking, it shouldn’t. While you can get a few laughs out of cultural shocks and misunderstandings, and perhaps a few gags about say, forms of address and the similarities of garum and fish/soya sauce, this would just end up making it another one of those series some people watch for the laughs, and then abandon without further thought as it degenerates into fanservice and plot hell.
No, what you need for a proper Byzantine anime is, as Soverihn has stated, people, characters, a real and living, breathing setting. Sure, it will be exotic, and there’ll be a bit of fetishization, but again, while that is the point, there’s also a difference. Great anime, in my very humble and inexperienced opinion, relies on highlighting humanity or aspects of humanity. Sure, you’ve got the harems and the slapstick, and those can be fun and all- but the really great ones are about who we are and what we want and think. In my humble opinion, something like Gundam is meant to meant to question the real-life applicability of simple ‘good vs. evil’; Neon Genesis Evangelion is a brutal deconstruction of the mecha genre, its underlying wishes and darker undercurrents; Madoka is likewise a brutal deconstruction of the ‘magical girl’ genre.
How then does this relate to the Byzantines? It relates through the fact that the Byzantines are so much more than just the woeful cloak-and-dagger stock villains they were reduced to- they were a mighty, learned, organized, and prosperous empire, to which European civilization as a whole owes a great debt. There like few other places temporal and spiritual power were intertwined, such that they were accused of caesaropapism, convinced of their divine mission; and yet it was never actually so- the throne had to bend, as saints took up not the mantle of government but of service to God, even if it meant challenging the former. It was an empire which reached a level of organization and administrative ability most other states in Europe could only dream of, and yet it too was plunged into vicious civil wars, bloody intrigue, and ruthless powerplay. It was one engaged in war after war, to defend its imperial legacy and Christendom, yet Byzantine emperors would ask absolution for those they were about to kill and had killed, and whose daughters would stare at the ‘flower’ of European knighthood in confused disapproval at such a term as ‘crusade’. They were driven, at least in theory, by an imperial and religious dream, a vision of God’s empire on Earth. The Byzantines encapsulate so much of what we think, and can recognize, yet whose take on some matters and own ideas impress and give food for thought; it makes us wonder that such men and women once walked those very same tiles of the Hagia Sophia. You have so much material for a properly great anime: dedication and fanaticism; family, friendship, and rivalry; love and hatred; ambition and power; cruelty, mercy, innocence, and blood; intrigue, ruthlessness, loyalty and virtue; military, civic and religious powerplay on an epic scale.
There are great parts of Byzantine history, which for all my usual literary/historical interest, could give anything in the Three Kingdoms or Ming-Qing transition a proper run for its money. In fact, considering how advanced and powerful the Byzantines were, I suspect a Japanese animation studio that does its homework, knows what it wants and wants something that’s good, might opt for a China-esque take on the Byzantine empire (which would be oddly fitting, considering the Chinese at first thought the same); except with the added bonus, of course, that they’re Byzantines, which as we all know makes it just that much better. And while you might have someone engage in gratuitous Greeku, in the way some characters engage in gratuitous Engrish in the style of Sengoku Basara (the infamous ‘Put ya guns on’), any really good anime, I expect and hope, would explore it much deeper than that.
I mean, wouldn’t it be awesome to see a properly done Basil II return at the head of a Viking host, and then afterwards turn north and east to battle against the great Bulgar and Fatimid armies? The Komnenoi, three generations of badassery, save the empire from the brink of disaster as they struggle with familial issues? Or even Constantine XI, knowing that the empire is probably beyond all hope, struggling to keep what remains in one piece, yet who refuses to surrender, as we see the defenders charge forward under the imperial banner, while the sun’s rays shine on the Hagia Sophia one last time?