This idea has been rattling around my head for sometime and it'll drive me crazy if I don't express it somewhere. I'm under no delusions that it will ever be implemented; this is basically a mental exercise.
My idea is for the implementation of additional classes, by changing the theme, aesthetic, flavor and damage types of an existing class, but keeping it's numbers and mechanics identical. The only class I've got a decent idea for is Warlock<==>Necromancer. How a character would toggle between the two I don't know. Maybe a quest-line like the green fire quest, or some sort of new glyph or something.
The way this would work is that all of a Warlock's Fire and Shadowflame spells are replaced with Frost and Shadowfrost equivalents. The Shadow spells remain the same, but some of them would be renamed to sound more necromantic and less demonic. Demonic Minions would be replaced with Undead minions that would function identically. And abilities, talents etc named after famous Warlocks and Demons would now be named after members of the Scourge.
A Necromancer's Specializations would obviously mirror the Warlock's. Affliction would remain largely unchanged with a few exceptions. Rain of Fire would be replaced with something like 'Frost Storm' and do Frost damage. But many of Affliction's themes would work equally well for a Necromancer.
Demonology would be replaced with Necrology. Demonic Fury would have become something more suitable to undeath. Metamorphosis would give you into a Lich instead of a demon. I have no idea what Dark Apotheosis would look like.
Destruction would change the most. I can't even think of a suitable name for it. A single word that would suggest frozen rot and corruption. Burning Embers would be replaced by Icicles. Emberstorm would become Frozensoul or something.
There are huge problem with this I know. For one thing, I doubt we have the sheer variety of Undead to replace demonic minions. Also thinking up Frost equivalents for so many Fire spells (Immolate, Incinerate, Conflagrate) is handicapped by the language itself; we have so many more words for destruction by fire as opposed to destruction by cold.
Also I have only the barest of ideas how this could be implemented for other classes.
Death knight<==>Dragon knight. Death knights created by Bolvar and empowered by Alexstrasza's flames instead of the Lich King. This would be the opposite of the Warlock<==>Necromancer. Frost spells are replaced by fire. The diseases are now Blood Corruption (Blood Plague renamed) and Burning Blood (Fire equivalent of Frost Fever). Your Gargoyle would be some kind of miniature Proto-drake.
Druid<==>Druid of the Flame. Replace Arcane spells with Fire.
Shaman<==>Dark Shaman. Nature spells changed to Shadow.
Paladin<==>Twilight Vindicator. Holy to Shadow.
Like I said, I know this will never be implemented. But it's here for your consideration anyway.