In many applications, a developer can define a "What's new" section and it is shown to the developer before / after updating. Eg on Android it is shown in the app description, in many browsers an extension can eg open a new tab showing new features, in sublime text a developer can show a message showing what is new after updating etc.
Currently, a developer is supposed to manage a "Changelog" section. Maybe we could show the changelog in a dialog or something after and/or before updating. I'd show it afterwards since it is kinda possible to see them before already by clicking on the plugin name and then opening the tab "Changelog".
It would be also nice to have the changes for Piwik core directly visible in the Piwik App and not on piwik.org (for a better user experience). This would be also useful for Beta changelogs see https://github.com/piwik/piwik/issues/9355