Yeah, there are a couple of people from Pivotal working on a column store right now, but a major limitation is that while the storage is now pluggable the query planner and the executor still assume all engines are row based which prevents PostgreSQL from taking full advantage of a column store.
Edit: Another project for a new storage engine is zheap which is a new row based storage which uses undo log rather than storing multiple row versions to reduce the amount of bloat.
Here are some slides from a presentation of Andres Freud on the matter:
https://anarazel.de/talks/2018-10-25-pgconfeu-pluggable-stor...
Slides 4 and 5 give some insight about the architecture changes which have been done.
> This enables the development of new table access methods, which can optimize storage for different use cases. The existing heap access method remains the default.
https://www.postgresql.org/docs/12/tableam.html
Interesting, this could allow purely in memory or columnar tables. A combination of postgres JSON features with an in memory or columnar table would be a great alternative to some nosql use cases.