You may know that optifine adds emmisive textures. AKA textures that glow. Would I be allowed to make a resourcepack that uses the vanilla textures and makes ones that should glow do exactly that?
It already exists elsewhere on the internet, and no it is not allowed here. Unless we hear otherwise from Mojang a strict reading of their literature leads me to believe that they do not want their work redistributed on the internet without their permission.
On the other hand, if you can managed to do this while not actually using anything from the default files then SURE! Go for it! As long as everything is your work and nothing is taken from default then that's totally fine!