What I'd like is stacking limited based on terrain to properly represent supply and food considerations for armies on march. Basically, if you have more units on a tile than it provides food for your units suffer damage similar to the Russian UP in RFC. You should always be able to have at least one unit on a tile unharmed, so this would only start taking effect when you put more units on it. So for example you could have one unit on Desert and Ice, two units on Tundra and Plains and three units on Grassland without suffering damage in the beginning of the game. This number should naturally increase with certain techs and civics, and with something like Refrigeration or Combustion I think it could be removed entirely. Furthermore, not all of your units suffer damage if there's too many of them, rather only a number of units equal to by how much you exceed the limit. There could also be promotions that counteract this.
Also I'd love love love if the culture system of 5 would be included, it's just perfect in every single way. Credit where credit's due, this is the one thing where it really shines. Apart from that it should be more similar to 4 than 5 with some new ideas thrown in as well.