I think Barbarian would work very well as a theme.
Ranger probably not, since it's so specific, but some other classes, like Assassin would make more sense as themes.
I have no problem with Assassin as a theme. That way you can be a fighter-assassin or a rogue-assassin or a wizard-assassin, etc.
In truth, any class, subclass, kit, prestige class, or paragon path that could be applied to any of the basic class archetypes could conceivably be a theme -- as long as its focus is broad.
Amazon comes to mind. How many 2e classes had amazon kits. Noble as well. Peasant Hero anyone?
Cavalier could definitely work as a theme.
Savage could work; however, barbarian has always been the D&D name, so I doubt it will be renamed.
Warlord. Yes.
Warlock wouldn't work; it's too specific, IMO. In fact, when you start to get into arcane and divine nomenclature, it gets a lot tougher. For example...
Can paladin be a theme? Can you be a wizard-paladin? A rogue-paladin?
What about sorcerer?
Now, elementalist, on the other hand, would be an excellent choice for a theme even though you'd have trouble applying it to non-spellcasters. But, imagine a fighter-elementalist. How could that work?
Lots and lots of roleplaying potential.
