It would be nice to be able to include user-defined metadata in replset config.  For example, one might include information like team/project owner, or a description/purpose of the replset.  This would be somewhat akin to $comment on queries.

You can sort of fake this with tags, but tags apply at the node level, and can't be applied to arbiters.  Tags are intended for making routing decisions (hence why they don't make sense on arbiters), so it feels weird to use them for this purpose.