Deal with new Japanese era
As most people in Japan should know by now, a new era of the Japanese calendar will start in May 2019.
It is not clear when the new era name will be announced, but it may be at rather short notice.
The Unicode consortium has made up a plan about how to deal with this, see http://blog.unicode.org/2018/09/new-japanese-era.html. Among else, they will encode an equivalent of the current ㍽, ㍼, ㍻, and plan to publish a special version of the Unicode standard numbered 12.1.
We should start to discuss and tentatively decide what (if anything) will be needed for Ruby. Alternatives include the following two and others inbetween.
Follow past years where the update to a new Unicode version is done with the Ruby version release around Christmas. That would mean we upgrade to Unicode 12 around Christmas 2019. At that time, we can include the changes in 12.1 without any problems.
Implement the upgrade to Unicode 12 as quickly as possible in trunk, implement Unicode 12.1 as soon as available in trunk, and backport the changes needed for the new era name to older Ruby versions.