Rather than a query planner, an interesting approach would be to expose the more stable part of the internals with a new language and let people roll their own query plans. Then Postgres can be NoSQL too and we can all be happy.

I'm not hopeful that it would be technically feasible, but it isn't obvious that Postgres needs to only support SQL as an interface. The SQL language is so horrible I assume it is already translated it into some intermediate representation.