It would be nice if ffmpeg and libav could be installed simultaneously. By default the library names conflict, and since a lot of packages depend on these libraries, and detect their names in different ways, getting linking correct in such a situation would be a pain. So I'll concentrate on the command line tools, namely ffmpeg vs. avconv. It should be possible to add some use flag which installs these packages with a prefix (or suffix) to their library names, so that the libraries don't conflict. That way, the flag could be set for one but not for the other, making both command line tools available while providing the common (un-prefixed) library names only from one of them. Of course the virtual/ffmpeg package should be modified to require that the prefix-enabling USE flag is not used in one of the installs. And the blocks would be for the non-prefixed versions. The prefix-enabled version would omit all the files needed for development: the headers, the pkg-config files, and perhaps even the *.so symlinks. You might consider re-using the libav USE flag. So ffmpeg[libav] and libav[-libav] would be prefix-enabled cli-only, while ffmpeg[-libav] and libav[libav] would be the full-featured versions. That way, changing a single global USE flag would change the dominant version all over the system. The main benefit of simultaneous installation is that users can compare both tools more easily, thus making a more informed decision when choosing which of them they want to compile their other packages against. Furthermore, developers can make their applications work with both as long as they use the command line interface.
if you can convince either (preferably both) upstream to have an option like --global-prefix=something, like --program-prefix on some GNU packages (eg findutils) that might work, otherwise I seriously dont see such hackery suitable for the gentoo portage tree
