I see this has been suggested a few times already, and I'm surprised it hasn't gotten the votes yet. It has several races that are very popular (Lizardmen, Leshys, and Hobgoblins) as well as interesting heritages for all the base races. Having these available for both players and DMs would be very useful imo. On top of those, it also has some interesting organizations such as the Hellknights and Knights of Lastwall (among others) that would be really nice to have available from the compendium. Compared to the books that have already been added it's also on the shorter side, from a work input viewpoint.