Really, when you look at the long-term viability of any platform, pluggability is where it's at. A lot of the success of PostgreSQL to date has been built on extensions and portability, just as the success of AWS has been built on their comprehensive API. Our future success will be built on taking pluggability even further.
In addition to pluggable storage, a second thing we really need is a pluggable parser interface. That is, it should be possible to generate a statement structure, in binary form, and hand that off to libpq for execution. There was recently some discussion about this on -hackers.
If there were a way to hand off expression trees directly to the planner, then this would allow creating extensions which actually had additional syntax, without having to fork PostgreSQL. This would support most of those "compatibility" extensions, as well as potentially allowing extensions like SKYLINE OF which change SQL behavior.
It would also help support PostgreSQL-based clustered databases, by allowing all of the parsing for a particular client to happen on a remote node and get passed to the clustered backends. The pgPool2 project has asked for this for several years for that reason.
More intriguingly, it would allow for potentially creating an "ORM" which doesn't have to serialize everything to SQL, but can instead build expression trees directly based on client code. This would both improve response times, and encourage developers to use a lot of PostgreSQL's more sophisticated features since they could access them directly in their code.
Taking things a step further, we could extend this to allow users to hand a plan tree directly to the executor. This would fix things for all of the users who actually need query hints (as opposed to those who think they need them), as well as taking efficiency a step beyond cached plans.
There are a lot of reasons this would be just as difficult to do as pluggable storage. Currently parsing depends on a context-dependant knowledge of system catalogs, including things like search_path. So I have no idea what it would even look like. But a parser API is something that people who hack on Postgres and fork it will continue to ask for.