First off this is not a thread asking for hybrid races in the game.
Given that the Horde and the Alliance are coalitions made up of the various races of Azeroth, some of these races long time members, others new, wouldn't there be more hybrid races in reflected in the factions?
Orcs and Trolls have shared a long history dating back to the Second War with Forest Trolls being part of the Horde and later on Darkspear Trolls. Similarly, Gnomes and Dwarfs have a close relationship dating back hundreds of years. Although in game we did see a marriage between a gnome and goblin in BFA. While this is not a priority, it would be cool to see in a novel that races of the Horde and Alliance have started to intermingle.
Obviously Me'dan was a complete flop to introduce mixed races in-game. Which is partly why it is not touched upon, but done correctly it would be interesting to see what the reaction of lore characters would be.
I mean an Orc/Troll Hybrid would be a scary sight. The strength and bloodlust of an Orc combined with the regeneration and berserking of a troll would be legendary.