I mean this insane idea I keep having. Somewhere in my brain meats, I want to run an old-school D&D game, probably using B/X or an associated retroclone (Labyrinth Lord or Lamentations of the Flame Princess; likely the latter out of personal preference, and because it suggests that the experiment would be more warped).
The setting doesn't really matter; maybe I'd use Greyhawk, or Blackmoor, or LotFP's implied setting, or even grim and terrible Athas. Or maybe I totally wouldn't care and it would be a homebrew. The whole adventure might take place in a megadungeon.
That's not the important part.
The important part is that there's a game, and its sequel.
The first game features fifteen characters — if there are fewer than fifteen players, then roles get doubled. Whatever the case, there are fifteen characters available: 1 halfling, 1 magic-user, and 13 dwarves.
The second game features nine characters: 4 halflings, 1 magic-user, 2 fighters (one of whom may be a ranger if the rules support it), 1 dwarf, and 1 elf.
In no other ways should the game directly reference Middle-earth (other than whatever typical D&D tropes happen to emulate Tolkien, of course). The goal is not to emulate The Hobbit or The Lord of the Rings, but to use the same party makeup and see how things go.
The only other Tolkien trope I might adapt is to attach grand significance to an otherwise "mundane" magic item found in the first game and make it the focus of the second game. That +1 dagger found on the first level of the megadungeon? It turns out it's Ichor-Spilt, Assassin-Blade of the Gods, and the gods aren't too keen on having somebody running around with the only threat to their power. That treasure map you never quite had time to decipher and only promised a few greasy coins, anyway? Well, actually, it leads to the Tomb of the Thrice-Damned Forgotten Lord, and that becomes the megadungeon setting of the entire sequel.
One game ends when all characters retire or die.
Addendum: It did occur to me that there is one small problem with this setup. No party cleric. I don't know whether I'd have the PCs tough it out, or whether I should just let the party magic-user cast from the magic-user and cleric lists. Tolkien made his magic-users angels or somesuch, after all...