Is there a process for creating a release? ie who would make this decision, and on what basis?
klembot is the main developer of the Twine 2 project and has final say on when a new version of the application is ready for released, and If you look at the Commits page of the project's repository you will see that the application was still being worked on as recent as 13-sep-2017.
In the past there has generally been at least one new release each month although based on information from the project's Downloads page this rate seemed to of slowed down as the last release was on the 02-may-2017.
There are any number of reasons why a new release can be delayed: from the developer not having the time to create one; to having to wait for a story format to be updated/released first (due to how they are bundled with the application).
...it is possible for someone to create their own release?
Chapel has given good advice on how someone may create their own version/release of the Twine 2 application.
If you do this and plan to use the SugarCube 2.x story format then I would suggest updating the version that is bundled with the application with the latest from that story format's project website (or it's repository).