The FHS has been poorly done for years. For e.g. where is the canonical place for a Web vhost root? /srv/www? /srv/web? /var/www?
One thing I don't like about its filesystem is its insistence that all binaries go into /bin. As a user, I greatly appreciate the concept of having some sort of /local directory to distinguish between files (or just executable in this case) which the end user installed and which are part of the distribution. If you abandon the idea that /bin has no subdirecories, then I could see /bin/local as a viable alternative (although I think I would still prefer a /local folder where that had /local/bin, /local/etc ...).
Debian Policy makes this more explicit by noting that it defines what packages (and hence: package maintainers) must do, may do, and must not do.
Among the latter: other than creating some of the hierarchy under /usr/local, packages may not delete content under this tree. That is for local system management.
Similarly, /opt as a location where third-party vendors can install their crap, excuse me, packages, is a pretty well established standard. Note too that you can offer the filesystem view independent of the underlying storage view, whether by symlinks (e.g., ln -s /usr/local/opt /opt), union mounts, or other means.
Any parts of the filesystem you create outside the defined standards are pretty much yours to worry about, though I've also got extreme reservations about polluting the root filesystem excessively. Better to create a structure under /usr/local or elsewhere (if for no other reason: it simplifies backups). This is a practice often respected in the breach in reality, however....