Here is a draft of roadmap that list of works that can be done for Proofreadpage.
Short term 
Finish and deploy the new configuration system for index pages : fully retro-compatible with the old one, small Wikisources can use ever the old configuration system (bug) (Tpt).Done. Finish and deploy the OAI-PMH API for index page, addition of new features (sets, new metadata formats (qualified Dublin Core...)...) (bug) (Tpt).Done.
- Make the needed internal changes in order to prepare ProofreadPage to work with VisualEditor: it's mostly a refactoring of the code of the current edition interface.
Medium term 
(the sentries are sorted by priority)
- Make VisualEditor work with ProofreadPage
- Write a full documentation of Proofreadpage feature by regrouping documentation written in mediawiki.org, wikisource.org and en.wikisource.org. Maybe in the help namespace of mediawiki.org
- Creation of a configuration special page in order to help people to install and configure ProofreadPage (bug). (Tpt).
- Continue to cleaning of internal code (already began by Beau) : index pages management (Tpt), zoom system (?).
- When the Mobile frontend will be merge into Mediawiki core, work on ProofreadPage outputs on mobile devices.
Long term 
- Finish the cleaning of the internal code.
- Use ContentHandler in order to make a more powerful storage for Index pages.
- Create a Lua API to get Index pages data. (depends on the precedent). This will help the community to create amazing new templates.
- Integration with Wikidata : use of Wikidata URI in order to build an RDF API for Index pages.