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Add a new reason that exceptions could be granted. When the app needs a newer version of the library and the library upstream has the necessary changes in their tree; they just haven't made a release yet. The following section would be added to Packaging:No_Bundled_Libraries#Some_reasons_you_might_be_granted_an_exception

Needing unreleased features

When an application needs unreleased features of a library and that library has committed to those features (usually, the changes are checked into the trunk branch of the upstream's revision control system) but the library has not yet made a release that has that code an exception may be granted to bundle that library until the Fedora packages contain the necessary extra features. Note that for this we would definitely want the standard questions regarding why the Fedora maintainer of the library feels we cannot include a backport of the feature/pre-release snapshot in our package, the timeline for the change to be merged and unbundling to occur to be answered so that we can make sure this is fixed should the package maintainer disappear or get busy with other things.

Not for bugfixes
As noted earlier, bugfixes are rarely a good reason for this. Most of the time bugfixes should be backported to the current Fedora package rather than bundling a library.