There are a bunch of limitations that, for now, limit how much I can do with regions. For one, building regions is not a cheap operation to do, performance-wise. So I want to limit how often those operations happen. In that regard it's kinda difficult to make regions more dynamic, but not impossible.
I agree though that regions should be more tied to cultures. And that's one of the reasons I want to split big regions into smaller chunks. Since subregions could be defined in accordance to the explorative capabilities of the cultures that discover them. Nevertheless, biogeography probably plays the most important role in how regions are defined in our world. And I want the most basic types of regions to be defined in those terms too. Just to clarify though, regions are not the same thing as territories (at least not within the game). A territory is 100% defined by the polity that controls it. And it can overlap or cover one or more regions, or just parts of them. I think your definition of dynamic regions and borders more closely resemble what I hope to achieve with territories. Hopefully I will be able to match your expectations better with territories once I get to improve them.
One change I want to make in this release, for regions, is to allow for them to be renamed, or rather, to have multiple names. I want each culture that discovers, or rediscovers, a region to give it a unique name in their own language. And, in the case of a single region having multiple names, I want the name to be displayed, at any given time, to be the one given by the culture that is more prominent within the region.
Anyway, thanks a lot for your comments. They made me reconsider my understanding of regions a bit more.