Hybrids HAVE been tackled in the game. If you've at least gotten near the end of the new story quests, you've seen a cross between Hyur and Elezen. They basically put Elezen ears on a Hyur female. All these races are humanoid, so it's pretty reasonable that they could produce hybrids.
Easy enough to allow features of both in a character creation system (I'm thinking we'd design these much like we design our retainers).
Or, we could have an option to adopt, much like in Skyrim. Think about it. The Calamity was 5 years ago, and HAD to have orphaned quite a few children. Bunch of 5+ year old orphans needing homes.