The Glen
Legend
Well let's be honest, who doesn't the Glantrians dissect? It's their formal introduction of choice.
The one thing I learned converting Mystara to Fifth was dealing with a large number of disconnects in the modules. So many of the gazeteers included bits of history or mechanics that are never mentioned or explained in any other sourcebook. Ethengar for example mentions how important the spirit world is in Mystara, even having a class devoted to them. However spirits are never mentioned again in any other source. Northern Reaches gives us runes and Modrigswerg, again only mentioned there. It's hard enough to balance the options in each gazeteer, trying to put them into 5E required some serious tweaking.
There's a lot in Mystara's history that gives it flavor but doesn't work with all of 5E. There are no drow at all. You've got shadow elves, but they share no traits at all. They aren't more magical, they don't take sunlight penalties, they aren't normally evil or even dark skinned. The Mystara dwarves as discussed are extremely magic resistant at the cost of being unable to use arcane magic. This makes them unique, they aren't just Forgotten Realms dwarves shoehorned into every setting. I don't mind the lack of variety as long as it pushes the narrative. Rockborn dwarves are borderline xenophobic, too often blinded by greed, and tend to fill the arcane engineer role normally filled in other settings by gnomes. Elves Mystara has in droves. You've got not just the elves of alfheim and the shadow elves, but you've got Belcadiz in Glantri, Water elves of Minrothad, the Vyalia in Thyatis and several others that you can use existing rules for. As far as the half races, in most of the canon half races require pretty much divine intervention to create an actual new race. Take the N'djatwa, which were merged by two different immortals. The half elves of the Savage Coast were effected by the Immortal powered Red Curse, keeping with the theme. The fact the races aren't very genetically compatable makes them much more unique than the regular settings. Settings without specific species is the norm in D&D, Mystara should be no different.
The one thing I learned converting Mystara to Fifth was dealing with a large number of disconnects in the modules. So many of the gazeteers included bits of history or mechanics that are never mentioned or explained in any other sourcebook. Ethengar for example mentions how important the spirit world is in Mystara, even having a class devoted to them. However spirits are never mentioned again in any other source. Northern Reaches gives us runes and Modrigswerg, again only mentioned there. It's hard enough to balance the options in each gazeteer, trying to put them into 5E required some serious tweaking.
There's a lot in Mystara's history that gives it flavor but doesn't work with all of 5E. There are no drow at all. You've got shadow elves, but they share no traits at all. They aren't more magical, they don't take sunlight penalties, they aren't normally evil or even dark skinned. The Mystara dwarves as discussed are extremely magic resistant at the cost of being unable to use arcane magic. This makes them unique, they aren't just Forgotten Realms dwarves shoehorned into every setting. I don't mind the lack of variety as long as it pushes the narrative. Rockborn dwarves are borderline xenophobic, too often blinded by greed, and tend to fill the arcane engineer role normally filled in other settings by gnomes. Elves Mystara has in droves. You've got not just the elves of alfheim and the shadow elves, but you've got Belcadiz in Glantri, Water elves of Minrothad, the Vyalia in Thyatis and several others that you can use existing rules for. As far as the half races, in most of the canon half races require pretty much divine intervention to create an actual new race. Take the N'djatwa, which were merged by two different immortals. The half elves of the Savage Coast were effected by the Immortal powered Red Curse, keeping with the theme. The fact the races aren't very genetically compatable makes them much more unique than the regular settings. Settings without specific species is the norm in D&D, Mystara should be no different.