I really do not like this idea. I feel like it just goes against the style of the game. I realize there are resource packs that go against the style of the game, but at least you choose to use those. With skins, people would just be waltzing around with HD-skins breaking the asthetic of the game. Also, 320x320 would be a terrible resolution, as it is not a power of 2 and therefore would not map very well onto the model.
i made a 320x320 skin, and this is awesome! and in my opinion, in servers it would be very cool, so much different skins... every day i see most of the skins identical!
I don't recommend these "loose changes", discarding the slime block mechanics making them act more like regular blocks and not allowing flying piston machines. We shouldn't let mojang remove the mechanics for the new block if they were ever going to remove it, like they did in the snapshot.