OK. I think I've come up with the solution, I just have no idea how to implement it simply. I think that in order to make this possible, each entity with a renderer needs to have its own custom renderer, which uses the custom model class for the particular entity (the class files could be placed in "<resourcepack>\assets\minecraft\models\entity\"). The game from there would need to interrupt the RenderLivingEvent and cancel it. From there, the game would find what kind of entity is being rendered, check if a custom model class exists in the directory mentioned above. If it exists, it would render the entity using the custom model and texture, but if not, it would render the entity using the vanilla engine.
Again, I think this is the best way to do it (after DAYS of searching through the base code), but I have no clue how to implement it, other than creating a custom rendering class for each entity in the game, which would probably cause tons of lag. Even then, I have no clue how to set up a custom rendering class, I just think that this would be the best option.
If anyone knows any ways of doing this, please post here. I would love to understand how this would work, even if someone beats me to a final product. I just want something like this to be made so I can make a resourcepack with custom mobs
all of this time I have been trying to get this to work, but I have run into another roadblock. Is there any way to get a list of all of the entity renderers? I want to do a for loop to add the default renderers to a folder if they do not exist, then use all of the classes in that folder. This folder is going to be in the resource pack that is currently active (Probably under assets/entitymodels). I just can't figure out how to obtain an iteratable list of every entity renderer. Any ideas?
“Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there-on a mote of dust suspended in a sunbeam.
― Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space