In a small way, I have succeeded in an experiment to get RootsMagic's automatic ShareMerge feature to work in conjunction with an Ancestry Family Tree so that changes and additions I make online will more accurately find their way back into my database. It's early in the trial but I'll throw it out for discussion.
RootsMagic assigns a UniqueID to each 'person' in its database. The actual value looks similar to '2A6784DDA70D43F1B5000A51906F073F2FE7', a hexadecimal string that has an extremely low probability of being duplicated for any other person in any other RootsMagic database on any computer. This value travels with the database 'person' when transferred to another database by drag and drop or by GEDCOM with RM-specific features included. It is the key to Share Merge:
So what if there is a way to get the UniqueID value into the Ancestry persons corresponding with persons in my database so that when I bring back the revised tree from Ancestry to RM, the latter won't mistake them for new people and open up that whole duplicate search/merge issue?
If you send a copy of your database to a friend or relative (either in backup or GEDCOM format), and they make changes to the database and return it to you (in GEDCOM format), you can import that GEDCOM into your database, and use this option to automatically consolidate the changes.
In my RM database, I created a custom Personal fact, RMID, Date: No, Place: No, Description: Yes, Include only GEDCOM and Lists, Sentence template: empty. Using SQLite, I added a RMID event to the EventTable for each person with the UniqueID for the person stored in the Details field (Description on the Edit Person screen). I did the same with the built-in REF# fact type. Then the database was exported to a plain GEDCOM (RM specific extra details unchecked).
This GEDCOM was uploaded to a new Ancestry Family Tree, where, to my surprise, the RMID custom fact appeared but the REF# fact did not. A note that I had included with each RMID fact gave rise to an "Unsourced citation" for each RMID fact. I made no changes to the Ancestry tree and downloaded its GEDCOM and imported same into a new database.
In this new database, the persons were assigned new UniqueIDs and therefore they cannot be ShareMerged when transferred to the original. Using SQLite, I replaced these new UniqueIDs with the RMID value for each person; now the persons in the database from the Ancestry download have the same UniqueID as the persons in the database uploaded to Ancestry.
I drag'd and drop'd from the Ancestry download database to the Ancestry upload database and ran the ShareMerge. Voila! Every pair of duplicate persons merged as did all the events, with the exception of ... RMID! Some more fiddling with its event values might correct its migration to not-exactly duplicate.
However, Sources are not without issues. The "Unsourced citation" created from the upload of the RMID Note resulted in a new source named "Source #n" for each person; again, I hope to obviate that with some tweaking of the RMID event. Worse is that the uploaded Sources come back different and won't automerge; there might not be any easy way around that but I have an idea... One impossibility I'm almost certain cannot be overcome is to get an online Ancestry source to return to Ancestry as an online source via GEDCOM through RootsMagic; they become user-generated sources in Ancestry with no links to the Census or other databases. Synchronisation a la Family Tree Maker 2012 is not on without an API - I'll bet that's where the _APID tag in the Ancestry GEDCOM comes into play.