With this command, you can change a player's texture pack. If the player doesn't own the texture pack that the command states, then nothing happens. But if they do have that texture pack available, then it will switch to that one. This would add a neat effect to some maps, if everything changes suddenly to look darker, happier, etc.
idk how effective this would be in practice, switching textures cuases minecraft to freeze for a minute, if mojang could optimize the texture change to be fast this would be a brilliant idea, but i guess for now we could set up loading screens haha.
You should also be able to disable the use of resource packs. The command should be /resourcepack <disable/enable> <Models/all/Resource Pack ID> <Number Place> Since resource packs can use spaces in its name, the pack will be assigned an ID (Like a UUID) if it has none. From there, you can enable that resource pack. The order place is like the arrows on the pack to move upwards or downwards, starting with 0 at the top. You would also be able to disable block models.