I wouldn't necessarily like a "sync all" ala FTM or Family Tree Builder, certainly not as the default, but I think lack of "batch" options makes the TreeSync impractical for large trees.
I would suggest greater flexibility, with some options similar the "Duplicate Search Merge" on the Tools menu. In essence, instead of trying to come up with "intelligence" to sync like FTM or FTB do, allow some selectable options to allow certain activities to take place en-masse during that "session".
From an operations standpoint let's start with "new" people on either side. After the Treesync scan some counts and options could be presented, i.e. xx in RM not in ancestry, xx in Ancestry not in RM, xx in both and indexical, xx in both and updated.
At that point some check boxes could be available like:
() Upload xxx new people in RM to Ancestry
() Download xxx new people in Ancestry to RM
() Delete xxx people from Ancestry not in RM
() Delete xxx people from RM not in Ancestry
Obviously some of those options are mutually exclusive, and the TreeShare options form should allow someone to set their preferred defaults.
This feature alone would assist greatly in synchronizing trees without requiring a fancy UI or AI. Not selecting anything reverts to the existing method, which is preferable when the volume of change is low.
You can extend this concept to updates. Let's look at a common issue for someone new to Ancestry or RootsMagic, place or name cleanup. You use the nice name or place cleanup tools provided by RootsMagic, but now you're facing thousands of clicks for all the events or people that changed. So some options such as:
() Update xxx Ancestry place names with RootsMagic places
() Update xxx Ancestry person names with RootsMagic names
A similar concept could be used for events, sources, etc, i.e:
() Upload xxx events in RootsMagic not in Ancestry
() Download xxx events in Ancestry not in RootsMagic
() Delete xxx events in RootsMagic not in Ancestry
() Delete xxx events in Ancestry not in RootsMagic
Again, these selections are in effect for that "session" allowing you to do each of those operations individually or not at all. Displayed counts could alert you if you were about to do something especially stupid. A summary of what's going to happen with an "Are you sure?" would de desirable, for example:
You've elected to delete xxx people from your Ancestry tree not in your RootsMagic database, download xxx sources from Ancestry and upload xxx sources to Ancestry. ARE YOU SURE?