If you want to publish your role on Ansible Galaxy or any other galaxy_ng instance, you will have to provide a MarkDown version of your README.
Changing the importer to allow other formats basically means that Galaxy won’t be able to render basic role documentation anymore, which is something I don’t think anyone using Galaxy would like.
If you want to use another format, you can either use a converter like pandoc to convert your README also to MarkDown, or you can add a small README.md file that links to the real README.