Thanks for your interest in the Region map visualization.
We are hard at work on a much improved mapping experience in Kibana. We will be rolling out a brand new Maps application in 6.7. This new application will allow customizing fill color, border color, border width, and symbol size. It will even allow for data driven styling allowing users to style each property based on values from a field or in this case, an aggregation metric value.
The existing Coordinate map visualization and region map visualization will be put into maintenance mode only and will not be accepting new features.