This idea actually came to me in a dream, and I further fleshed out the dream idea in a semi-awake state last night, and somehow managed to actually remember it this morning. I'd been reading a few blog posts about sub-classes and kits (or maybe it was on G+), so those obviously inspired this whole thing.
Take Classic D&D (OD&D, Holmes, BX, BECMI, RC) or any of the clones based on these (BFRPG, LL, S&W, maybe some other near-clones as well). Most of these have seven classes: Cleric, Fighter, Magic-User, Thief, Dwarf, Elf, Halfling. Most of you should know this by now, but you never know.
A simpler way to add some of the AD&D/3E/4E style classes to the game (and I play in a group with mixed preferences, so this is an issue for me) than creating a bunch of actual classes would be to rethink a few of the past choices of the game designers and use a version of the 2E AD&D kit to add a few things and take away a few others to customize the basic seven classes.
In my dream, I was tinkering with the Cleric class, so I'll use that as my example here. Someone wants to play a Paladin (or Avenger, Sohei, or other "church knight" type). Traditionally, this has been done by grafting some clerical abilities onto the Fighter. But why not just give the base Cleric class access to all weapons, at the expense of, for example, a slower Turn Undead progression and/or a truncated spell list? Clerics fight near as well as Fighters, and with the smaller hit die the new Church Knight variant Cleric isn't as likely to overshadow the Fighter, the way a Paladin traditionally does by giving it everything the Fighter has PLUS Cleric stuff.
Druid can be done with a simple reworking of the spell list and changing Turn Undead into Turn (or Calm) Animals. They don't really need animal shape-shifting and secret languages and all that, do they? And the hippie no metal armor, but metal weapons are hunky dory thing? Drop it or keep it as you like. It's flavorful, like the Clerics only using blunt weapons thing, but IMO unnecessary if the spell list has the right feel. And if the Druid spell list is being built differently from the standard Cleric list, why not just make a special wild shape spell similar to the Polymorph Self spell but limited to natural animals (and maybe plants?). Other AD&D/3E style Druid powers could likewise just be spells on the list.
Other ideas would include a Bard that is just the Elf Class without the racial abilities but with a bardic lore ability (more or less what I originally did with for the Bard, before reworking it to be more of a Cleric/Thief hybrid). Or, as others have suggested before, Tolkien style Eldar could be made by using the Elf class as-is, except giving them Cleric spells instead of Magic-User spells. Take the Fighter, up the hit die and restrict the armor and you have a loin-cloth Barbarian type.
OK, so not amazingly groundbreaking ideas here, they've been around before. The thing that was different in my dream was that I was working up a retro-clone where these were hard-coded into it, like the Profiles in Flying Swordsmen. Or maybe it was a stand-alone supplement that could be used with any of the systems listed at the beginning of this post. It was a dream, dreams are weird like that.