Since this touches the same subject, I'd like to suggest something similar to what I said here
One of the things preventing phpBB to run in cluster ("cloud") environments is the way it depends on the file system. There is no easy way to change the storage of files. And avatars are also files. Of course, one could just use remotely hosted avatars, but it's not the ideal solution.
By providing an abstract avatar interface the avatars could be very extensible, allowing integration with third-party services such as gravatar -- something I'd love to see, by the way.
Backward compatibility would probably have to be broken, however.
EDIT: I see this has also been touched in the ticket you linked to.