Okay here is the new thing I'm working on:
- city culture can extend no further than the third ring
- cities gradually cover the tiles around them as their culture increases (no wait for border pops on new culture levels anymore)
- more culture required to fill the entire second ring
- properties of the plot influence how much culture is required to cover it: more for instance for Ocean, Desert, Peaks, less for tiles with bonuses, floodplains or river connections
- tiles get covered according to their cost (cheapest ones first)
I made this change for a couple of reasons:
- the old rules were intended for BtS where cities could be placed a lot further apart
- the second ring could be reached too easily (10 culture is nothing) and after that culture was usually irrelevant
- cities with a lot of culture could cover vast areas of the map, especially with the fourth ring
- the gradual coverage makes up for the increased overall cost, it might take longer for the entire third ring, but the more useful tiles become available earlier (also it makes for more realistic borders instead of BFCs everywhere)
- dynamic tile costs are used to make sure useful tiles are available first (bonuses) and that borders follow geographical situations more realistically (for instance, deserts are covered last)
I still have to fine tune the costs and deal with some unintentional behavior, but in general it works well already.