Allura has a solid framework for doing imports, and has implemented this for services like GitHub, Google Code and Trac. It covers aspects from Git/Hg/SVN to tickets and wikis.
It would be great to enhance this to support importing & converting data from Bitbucket and Gitlab. The level of integration & conversion may vary depending on the format and structure of the services.
Beyond that, related improvements could be made in supporting ongoing synchronization of Git/Hg/SVN repositories (not just a one-time import). Or importers for JIRA, or improving importing of Allura's own formats.
Everything in the ForgeImporters directory is our existing import system. The forgeimporters/base.py file has several base classes that provide common functionality for all importers to use. In the subdirectories "forge" "github" and "google" there are specific importers that can be good examples to reference. Importers can also be external packages, see these repos as more examples: https://sourceforge.net/p/mediawikiimporter/git/ci/master/tree/ and https://sourceforge.net/p/tracwikiimporter/git/ci/master/tree/