- The scales of Civ5 and Civ6 are different ...
Civ5 has 1-tile-cities and so allows small island cities. Civ6 has multi-tile-cities which need a lot more tiles to work. If you like to play on real world maps, the Civ5-scale works on maps like Ynaemp (until you hit the 32-bit-memory-limit). For Civ6 they need to greatly increase map size to allow a playable world map since many civs like England or Japan start on rather small landmasses and so are handicapped by the new multi-tile-city-system.
Civ5 looks more like a world map while Civ6 looks more like a region map ...
- Civ5 allows an all wonder city (when located at the coast near mountain near desert tile ...), Civ6 does not ... unless you expand and have 10-20 cities, you probably won't have enough space to build all the wonders in Civ6.
- It might be too difficult to defend the multi-tile-city against raiding barbs or other civs in early game ... if you loose all your buildings in early game too often, you will likely go back to Civ5 ... (There was a reason why cities built city walls in history and placed the valueable buildings inside and not outside for a long time ...)
- Civ6 might be too short-paced with shortened tech-tree and eurekas so you might finish a game in a few hours
We will have to wait for Civ6 and try it out ...
From what we know so far, Civ6 will be a different game compared to Civ5. Both share a certain overlap (the civ-theme with civs, techs, buildings, wonders, units) but will also have big differences in game rules (e.g. districts) so Civ6 will not replace Civ5 but will more likely be just another new interpretation of the theme ... the biggest problem for the player will be limited time to play ...