I have been pondering this question for a little while. My lovely spreading "Chestnut Tree" has turned into a "Banyan Tree" as more cousins want to see more of their family in the tree. I would like to publish online separate trees for different family lines while still maintaining in FTM a single object for each person and each relationship. Access could then be granted to specific family trees without revealing details of others that they are only casually linked to.
The solution could be implemented in FTM by creating a virtual tree (equivalent to a "database view" for the cognoscenti). Each virtual tree could then contain one or more persons and all relationships between those persons. Of course, each person could be in zero or more virtual trees.
A problem this might create is conflicting updates to FTM when syncing different virtual trees. Could be solved by allocating each person or relationship to a "home" tree whose updates would prevail.
Now that is something that could be released by 2016(?). I wouldn't mind paying again, but in the meantime I think I need to support my own website.