I think it's worth writing down ideas, open questions, current unsolvable problems, etc in a document for future consideration on TinkerPop 4. While not on the radar as something to start coding on any time soon, having a place to stick these things for ongoing thought is better than just saying "eh, TinkerPop 4 problem" and just going about our day.
I created a new asciidoc book and started the document with something Marko wrote to the mailing list about ReferenceGraph a few days back. I don't really think there needs to be a format to this document or anything. It's basically just a scratchpad of ideas that I assume will grow over time. http://tinkerpop.apache.org/docs/3.3.0-SNAPSHOT/dev/future/
