I'm sure EVERYONE (Almost everyone) wants these two features:
-Update to 1.12
-Ability to add custom items/mobs/blocks without sacrificing an existing texture.
I am not sure if this is possible/realistic, and if it is not I understand.
you can add new custom blocks/items already into minecraft you use the client-side mod. Just Its in the version 8.8
The client-side mod is no longer working, unfortunately.
At the moment it's not possible to make custom mobs or items (though you can alter the texture of an item in the texture editor and make items using recipe blocks).
I know and understand that.