Marko Lindqvist wrote in #note-4:
Would next natural expansion be MaxTerrainClassPct requirement, to unhardcode ocean <-> land transformation rules?
With topology requirements to determine whether we're hex or not, plus a bit of crunching numbers, this is already possible with this (albeit less convenient). Oh, except at the edge of the map maybe, haven't checked how that's handled currently. So we might need that.
Then again, part of this whole #588 operation I'm doing right now is in the hopes of one day unhardcoding and allowing more than two terrain classes (early uses would be putting Lake and Inaccessibel terrain in separate classes). At that point "at most 85% the current terrain class" would no longer mean "at least 15% the new terrain class"; our unhardcoding efforts now might lead to a world where we can only express the former, but not the latter, which is what we really want. So maybe we should hold off on unhardcoding that until we have a satisfactory solution.