For merge rules, it is currently fixed which of the two records involved is primary and secondary. However, for certain scenarios, it would be very handy to have the possibility to change that default configuration (a) as default for the specific rule, and (b) when applying the merge rule in the metadata editor. For example, if you want to use external search to enrich your record on the left pane with the record on the right pane. This functionality would also facilitate the re-use of merge rules (say, you want the same kind of fields to be retained for an import profile and for an external search, then you currently have to write two rules - for the import profile with the existing record as the primary record, for the external search with the external record as the primary record).