Increased support for resource packs

In recent snapshots Mojang has added much more support for datapacks, giving them the ability to add entire new biomes and dimensions (though we still can’t use NBT in custom crafting). All this is great, but I feel like resource packs are falling by the wayside. We can animate and randomise block textures with the blockstate and model files, but those files only exist for blocks (and items for models). There is no such thing as a model file for an entity at an entitystate file. Basically I’m asking this because I want to make my zombies be 50% Steve and 50% Alex without Optifine