Mystara also had the most interesting take on elves and it had nothing to do with adding yet more subclasses.
There are two elf clans introduced in GAZ1 The Grand Duchy of Karameikos. One is close to bog standard D&D elves the other is given almost no description (left up to the DM, I suppose) but are much more secretive and have green eyes and red hair.
GAZ3 The Principalities of Glantri (probably the greatest setting supplement for D&D ever) introduces Flamenco elves and tree hugging hippy elves from Alfheim.
GAZ5 The Elves of Alfheim was written by Steve Perrin who is most well-known for creating RuneQuest. But he also created the RPG based on the Elfquest comic book. There are eight clans listed as well as a very in-depth look at Elven history that touches on why the elves are in Alfheim and describes some of the other elves that split off (like the two clans in Karameikos). The clans are basically what you'd expect (xenophobes, traders, lore, hunters, etc.) but there's enough there to give variety and, most importantly, inspiration for DMs and players. It gives motivations and allows for intra-elven conflict that doesn't devolve into armed conflict.
GAZ9 The Minrothad Guilds introduces Water Elves. These aren't elves that live under water (those would be aquatic elves) but elves who are sailors. They use their love of wood to build amazing ships that approach 18th or 19th century sail boats for speed and design. This is probably the only D&D setting I've seen that has such a thing.
GAZ13 The Shadow Elves introduces Shadow Elves that are Drow but better. They aren't irredeemably evil (though the surface elves think so). They aren't really even evil. The Gazetteer goes to great lengths to explain why they despise their surface brethren. Their patron Immortal isn't even an elf. It's a human who was obliterated in the Blackmoor Catastrophe.
The Hollow World introduces the Schattenalfen (German for Shadow Elf) who are Aztec influenced. Don't think too hard about why or how that happened. It just did.
If you are an elf in Mystara where you are from makes a big difference. I've had so far in my current Mystara campaign a Vyalia elf (the reclusive red-haired/green-eyed elves from Karameikos. Though this one was from Thyatis and worked for the elite Foresters), an Alfheim elf, and a Flamenco elf from Glantri. They all played differently because of their background and, most importantly, I as a DM had NPCs react wildly different based on their background.
And NONE of that difference is because of some mechanical subclass difference. It's probably the thing I hate most about races in other D&D - the proliferation of elven subraces to cover every stat out there.
I mean, to take some other example mentioned above about the invisible moon. Populated by samurai cats. It's so silly I had to incorporate it so I invented an elven ritual that takes place on the Summer Solstice (which I think is the first day of the six month of the year). The first of the month is always a new moon. I decided that on that day if you stared up at the moonless sky, looked in the right direction, and closed your eyes then at midnight you could see the invisible moon.
Yes, Mystara started out as a bog-standard fantasy setting but it really showed just how much you can stretch it. The master-class in elven design is just one example.
