Since Civ VI has blatantly modeled itself on boardgames, they could take a lesson from one of the best boardgame designers, Frank Chadwick. He and his Game Designer's Workshop crew put out an 18th century warfare/political boardgame decades ago called Soldier Kings. It had two mechanics that could be very useful: first, unlike most boardgames, there were no tiles, squares, or any other such feature on the map: all movement was point to point between towns and cities, so that the map consisted of a mass of city-points connected by lines representing 'roads' and all movement was strictly town to town and all battles took place either as sieges of the towns or field battles right outside the town.
In addition, to move more than one unit at a time they had to be grouped under a General, so the number of Generals was the number of Armies that were not immobilized in towns as garrisons. This idea has been used elsewhere, but given Civ VI's Great Generals and the arguments over stacking versus 1UPT, it's an idea that might be worth revisiting to thread the needle between Stacks of Doom and Scattered All Over the Map unit movement and combat.
The other way to get the 'no-tile' Appearance would be to use very small tiles, so that virtually everything on the map takes up more than one tile. This allows the kind of variation in terrain and infrastructure that looks more 'natural' than a larger and obvious system of tiles which, no matter what geometry you use (square, hex, dodecahedron, etc), still present an obvious regular grid of some kind. See the Anno 1800 game for an example of how 'natural' a map like this can look with modern computer graphics (and, by the way, that game seems, with all its 'natural' look, to present about the same degree of difficulty to my computer that Civ VI does, so the different appearance doesn't have to require another new generation of machinery to display it).
Either of these concepts would, of course, be a radical design change from any Civ before them, so they would have to present a whole bunch of advantages not available from any 'conventional' Civ Map to be adopted.