How about providing more granularity to replication, by setting separate values of replication parameters to each standby
for example:
standby1.wal_sender_timeout= 50s
standby2.wal_sender_timeout= 40s
The idea is to allow configuration of standby servers such that they have there own set of replication parameters as per requirements.
How does this interplay with the synchronous_standby_names parameter ? Or do you think that becomes irrelevant if we do like what you are suggesting above ? AFAIK synchronous_standby_names was added because we wanted to give master control on which standbys can become SYNC standbys and in what order of priority. But if we do what you are suggesting above, I wonder if we can just have an explicit knob to make a standby a SYNC standby. Say something like: standby1.sync_mode = SYNC|ASYNC.
The other purpose of synchronous_standby_names is to define order of preference for sync standbys. Even that can then be explicitly specified using this mechanism.
I am not sure but may be we can even allow to set synchronous_commit at each standby level. So depending on the synchronous_commit setting of each standby and their priorities also explicitly defined by this mechanism, master will decide which standbys to wait for at the commit time.