Civilization 6

One of the most criticized features of the Civ 5&6 is "one unit per tile" (1UPT) limitation - in the older versions of Civ, it was possible to stack an unlimited count of units on a single tile.

The benefit of the change is more opportunities for tactics. For example, it is possible to replay Battle of Marathon in Jadwiga's Legacy scenario - a single well placed pikeman can indefinitely block a mountain passage against a hoard of knights, till only a single knight can attack the pikeman per turn. Or you may place a city between two large lakes, like real-world Madison-Wisconsin, making it difficult to "flank" the city and put it in to the siege. And of course, 1UPT fixes the infamous Stack of Doom issue.

On the other end, it makes logistics difficult. It happened to me multiple times that I ordered a unit to move forward, only to watch it in horor how it moves backward because all the "resting places" in the forward direction are already occupied by my units. A simple mitigation of the described nuisance is to introduce a keyboard shortcut particularly popular in Microsoft Word: Ctrl+Z. The implementation can take an inspiration from The Battle for Wesnoth turn based strategy, which gives the user the opportunity to take back a miss-click. But to prevent the abuse of the feature by the players, it is not possible to take back a move that uncovers part of a map or a move that lifts fog-of-war. Westnoth also prevents you from taking back a move, which results into a combat. Since Civ 6 still uses randomness in the damage calculation, this limitation would have to be ported as well. The long loading times because of a silly miss-click would be mostly just part of the distant past...

Another irritating thing is that sometimes a victory (or loss) is inevitable. But you still have to get thru many turns to get there. Some people enjoy this part of the game. But I find it boring. It would be nice to have something like "auto combat" or "quick battle" from Heroes of Might & Magic III that would allow you to watch the march to the victory from the resting position of your seat or that would allow you to just skip to the victory screen because you really need some sleep time. Didn't you get the outcome you expected? No worry, just press the sweet Ctrl+Z combo and you can show the computer how it should have been done.

The biggest advantage of having the "fast end" option is that as a developer you do not have to stress yourself so much with fine-tuning the winning conditions. Does the game slip too frequently into the war-of-attrition? Just add "fast end" option. Problem mitigated. Release the game. And leave the re-balancing on the expansion pack.

And finally, the game logic for city states and civilizations should be the same thing. City states are great. But I want to be able to:

1. Befriend not only city states but also civilizations with amenities.
2. Get unique bonuses not only from being suzerain of city states, but also from being a friend to civilizations.  

And reversely, civilizations are great, but I want to be able to:

1. Select not only civilizations that appear in the game, but also city states.

All these issues can be nicely resolved by adding a switch into the "create new game" GUI next to each civilization, which would "cripple" the selected civilization to a city state. This switch would:

1. Remove the leader and her/his traits. But the civilization traits would stay there.
2. Limit the count of cities to one.
3. Adjust things like loyalty bonus,... to actually make the city states work.