The thing that I strikes me as odd about "reunion" is that it's only a reunion from the soul's perspective. Personality doesn't remember all those folk from past lives. So what's the theme of the project as personality sees it?
That could be said of all of the themes. In fact, it could be said of task companions, too. How many people remember their task companions from past lives? Yet they work together.
(For that matter, how many people remember anything or anyone from past lives?)
As far as the theme goes, it says that Kingdoms are the created gatherings, focus, and work of fragments who share a common, specific goal within the physical plane across time. Isn't that what task companions do, work together on a goal?
My perspective is that it's not for personality, it's for essence. And that working together on whatever theme is chosen results in the reunion of fragments.
Unless I'm missing something, I don't understand your problem with the reunion theme. I thought that was a given, that essences would want to reunite. Isn't that what we all do as we move through our evolution and progress to the Tao?