There are definitely two clear places where additional help would be useful and welcome right now.
Here's a shortlist of replication-related open items I put together earlier. These are all independent things it'd be helpful to have when working with logical replication in PostgreSQL. There are a lot of medium-sized pieces of independent work still to be done to make some things work well.
For example, you can't have multimaster logical replication with physical failover nodes for each MM node unless we replicate slot create/drop/advance over physical replication and copy them with pg_basebackup.
Handling big transactions better:
* logical decoding interleaved transaction streaming as discussed earlier
Automatic DDL replication:
* DDL deparse extension (Álvaro started some work on this)
* Better way to link the pg_temp_nnn tables generated during table rewrite to the original table in decoding
* logical decoding support for prepared xacts (before commit prepared). Useful for DDL replication, other consensus operations.
Using physical replication/PITR with logical replication:
* replication slots replicated to physical standbys ("Failover slots")
* pg_basebackup should copy slots
* slots should follow timeline changes
Failover between logical replicas:
* logical decoding support for sequences
* logical decoding support for slot create/drop/advance
* Separate slot WAL retention from confirmed commit point so you can say "I've replayed up to 1/0000AB but you should keep from 1/000001". Needed in async MM to cope with node loss properly. Will write it up separately.
Multimaster:
* Sequence Access Method
Other logical decoding enhancements:
* Support exporting a new snapshot from an existing logical slot. Useful for COPYing new tables not previously replicated when added to a replication set, for resync'ing tables, comparing tables, etc.
* WAL messages. Useful for voting, replay confirmation, etc. Rendered largely unnecessary by xact interleaved streaming.
Misc:
* An API to enumerate currently registered bgworkers
* A way to securely make a libpq connection from a bgworker without messing with passwords etc. Generate one-time cookies, sometihng like that.
* (unimportant but nice API fix): BGW_NO_RESTART_ON_CRASH
