We already have a real time changelog. You can always view our forge at any time. When we make changes we always create a forge ticket for them unless the change is something minor like fixing a typo in a string. There are plans to further improve CB Package Builder though so we can use API to grab the changelog instead of just sending to a projects roadmap page.
For nightly builds they'll usually always contain the next versions tickets that have been marked resolved unless they're still pending review. So for example the current changelog link for CBSubs is as follows since it's the latest stable.
forge.joomlapolis.com/projects/cb-paid-subscriptions/roadmap?completed=1#4.5.0
If you scroll down to 4.5.1 that will show you what's in the current nightly. On the roadmap new tickets are always at the bottom, but on the issues page below they're always at the top.
forge.joomlapolis.com/projects/cb-paid-subscriptions/issues
We'll be improving all this with CB 3.x since we've to change our versioning to be more compatible with Joomlas version checking since CB plugins will become Joomla extensions. We'll probably just have to release them as RC releases so version comparison can work better and being versioned releases we can ideally have specific changelogs for them.
What will also help is a better ticketing system. I'll be working on a new project eventually that just integrates with our Gitlabs so we can use its ticketing system and everyone can view tickets directly on Joomlapolis instead of being sent to our forge, which is just using redmine.