To me, beautiful military units are more important than beautiful terrain and buildings.
Unfortunately, Civ 6 has the major problem of ugly graphics of military units compared to Civ 5.
I sincerely hope that Civ7 will improve the ridiculous "Nanuchka class corvettes are missile cruisers" and "B-52s are stealth bombers" situation.
Yeah, I would spent hours upon hours when I was a kid, painting Egyptian ancient units, Barbaric units (Barbaric derives from the root word BARBA, or affrication of ARBA, which means ALBA, or DAWN in English. Related to the Sea People by the greeks, as the people that worshipped the SUN, and spoke the ancient root languages. Later Roman adopted Barbara term for all civilisations not Greek or Roman or Egyptian, and it then stuck to vaguely define cultures from Africa, Northern Europe, Central Asian steppes, etc) barbaric units had a multiplicity of colours... Red capes, Circular shields, Oval shields, Rectangular, with all sorts of designs like Medusa heads, eyes, crosses, pelts of all animal kinds... Blue Mayans, Green Amazonians (dressed with leafs) Wood dresses (Africa)... So much joy painting those little plastic units... It would be a work of love to introduce a painting editor in the default game, where everyone could personalise its little tribal civ colours, dresses, and later coats of arms...
The most sad thing in Civ 6 were the city states... giving identical units everyone the same... that I really hope will end... I'd prefer no units at all from city states than have to look at Venetian and Genoves units looks the same as Mohenjo-Daro or Hong-Kong... Singapore...