Hi,
here some other ideas:

• Allowing wildcards (,%) to ban / allow all e.g. minecraft: mobs (-> "minecraft:")
• Add whitelist and make them switchable in the configuration (to select whitelist or blacklist mode)
• Add command to morph/demorph players as admin (and usage permissions)
• Add command to morph a player directly to a mob/block/NBT data (and or to that what he is looking at)
• Add config-option to change morph animation time.
• Idea to the white/blacklist: the actual morphs.json file is the new white/blacklist file, here you can add mods with the option to mark them a whitelisted or disable them (blacklisted). In the config you needed to set which mode will be selected. If you understand what I mean :)

• Using /metamorph reload doesn't work in singleplayer mode

Nice mod!
Simonmicro