JDK 11 updates will be delivered in a quarterly cyle. Usually releases happen mid of January, April, July and October.
The mercurial repository jdk11u-dev mercurial repository is the standard default repository for collecting changes. The jdk11u repository will be is used for stabilizing and delivering the quarterly JDK 11 update releases.
Fixes in to the OpenJDK should must generally be done upstream in the upstream development repository jdk/jdk first. As a matter of fact, changes to JDK11 updates will mostly be backports of issues from upstream. There are exceptions when there's the need for a fix that only applies to JDK 11 updates.
Everybody should feel encouraged to suggest updates fixes for JDK 11 updates and do the work to get them in. Everybody can do it, at least for the most parts of the work. For details, on how to do it, continue reading here.
Should you, for some reason, not be willing or able to drive a fix into OpenJDK 11 updates, you can still suggest changes by dropping a mail to the jdk-updates-dev and askingmailing list. But by only doing only that, you are at the grace of some community member to pick it up and do the work for you.