I've given this a bit more thought and I believe the feature could be implemented using Trackers.
Trackers can be configured on a project to project basis as well be applied to every project.
If the Trackers had the option for a "Default-Description" which would fill the description of an issue based on what Tracker is selected, that would pretty much meet all my requirements.
The only option I'd really like to see is, that the Tracker is not pre-selected, so that it forces the user to take an active step in selecting the correct Tracker, but this could possibly be optional?
I'd like to be able to create templates which can be configured to be applied as default behaviour for every new issue or subtask in any project.
Especially the description would be of great importance.
The magic button does not quite cover the problem as it is "its own" button instead of replacing the "New issue" or "Create subtask" option.
I do not want to introduce a second workflow over the intuitive one, rather than optimizing it.
With multiple templates set up and configured for a project, an option to choose which template should be applied for the author would be required.
A good example of a setup I'd like to be able to do would be Magento's Github issue page: https://github.com/magento/magento2/issues/new/choose