GNOME forks Cinnamon and Mate (active), libav vs ffmpeg (now closing), mplayer/2/mpv (mostly overtaken by last one).
Most common pattern is multiple packages solving the same problem, rather than forks - and incompatible versions.
The rest are good examples but of the millions of projects very very few are forked successfully
I could add a couple to this list as well but that does not change the fact that successful forks are exceeding rare
GNOME forks Cinnamon and Mate (active), libav vs ffmpeg (now closing), mplayer/2/mpv (mostly overtaken by last one).
Most common pattern is multiple packages solving the same problem, rather than forks - and incompatible versions.