Even though I'm pro-Mystara, I'd personally I'd first like to see a Spelljammer+Planescape+Chronomancer* meta-setting book, which includes a planar/galactic map of all the worlds and planes ever mentioned in any D&D product, along with world-hopping adventure ark which visits a dozen of those worlds.
I'd be happy for Mystara to just be one of many adventure locales in a world-hopping uber-setting: the D&D Multiverse.
I would rather they pulled their PDFs from stores again than **** on their predecessors' work this way.
Thank you. That is perfectly said. Those worlds are D&D worlds, both in the system played and the greater multiverse they exist in.These worlds were connected from the start. The Mystara gazetteers had appendices with gates leading to Oerth and Toril. The original 1st Edition Dragonlance hardcover had extensive rules for what happens if a character from another world comes to Krynn. All the worlds were mention in Planescape, and had cameos from all worlds. Athas of course was cut off...but obviously its solar system lies within the D&D Multiverse, and there have been Dark Sun characters in Planescape. Ed Greenwood had the Egyptian and Babylonian pantheons actually travel from Earth to Toril. There was the Wizards Three series of articles.
Of course your position is a valid preference, and reflects occasional statements from some of the world-specific TSR authors such as Weiss and Hickman, but your preference is different than what the published D&D Multiverse has been in every edition. Same for 5E: the PHB lists all the D&D worlds as being located in the same Material Plane.
Presenting all the worlds of the D&D Multiverse as a single brand is not so different than the rebranding of the lands of Al-Qadim, Kara-Tur, Maztica as a single Forgotten Realms brand. It doesn't destroy the tone of Zakhara, Kara-Tur, or Maztica to highlight their connections with Faerun.
These are not pristine, haute-couture "art-worlds", or singular literary worlds like JRRT's Middle-earth. It's D&D!
Focus on what made each setting different so the players can choose how they want to play. Mystara is very much left for the players to discover. Lost Cities, hidden valleys, Borderlands, secret societies, and all of this before you reach level 4. How many of the nations are completely settled? Ierendi is the only one I can think of everything else has vast expanses of wilderness. The fact that players were expected to rise in their social ranks by becoming members of nobility was written into the rules. that was something that was missing in a lot of the other settings.
AD&D2nd Ed Mystara made two important mistakes:
a) TSR decided that Mystara should be presented as a kind friendly setting. (Not even kids want a kiddie setting).
b) TSR decided that in order to make Mystara as compatible with 2nd Ed as possible, they would tone down all the features that set the world apart from the other AD&D worlds.
That makes me think. Since Birthright and its domain management rules were not very popular, maybe they could mix add the domain rules to Mystara, since the rules and lore make a lot a place for social advancement and the place of heroes in the world politic. The Big Book of the Known World could be this type of D&D: the one where your adventurer gets to epic levels and rules for titles and domains. If Eberron is the setting where magic items are a common reward/currency, maybe Mystara could be the setting where renown and titles are as common.
I know the DMG has the some basic rules for building, renown and alternative reward, but I think a specific setting could expand them, just as I think Darksun could be the setting where exploration and survival rules are expanded.
I actual prefer Mystara with restrictions based on its history. Dwarves don't do arcane magic because that's the only way they survived the Great Rain of Fire. The dwarves that could are either in the Hollow World to save them, or are the Rot Dwarves trapped in their mountains in the Northern Reaches and cursed with insanity. There's a lot of Mystara that differs from Forgotten Realms which causes some conflict with Fifth edition.
I've used lots of information that Havard and The Glen (who I assume is Glen Welch) have published at Pandius or YouTube.
It's easy enough to take point #6 in Havard's post (a world where the strangest races feel right at home) to, for example, justify adding Tieflings and or Dragonborn.
Mystara was very additive and only rarely subtractive. It added all sorts of races, classes, nations, to the game. There's even the only example of fantasy Dutch I can recall. And it's a horrible pun as they are Followers of the Flame - Flaemish. So adding Tieflings or Dragonborn makes sense. There are even Diaboli that you could refluff as Tieflings if you wanted to.
The rare subtractive elements are justifying why half-elfs don't exist and why dwarves/halflings can't cast arcane magic, for example.
The setting also does a great job of mimicking 5e's Tiers of Play, which roughly follow the BECMI boxed sets. You see it in Havard's point #2 (Room for both low level exploration and plane hopping world altering epics.) The modules were forced to accommodate the boxed sets and the Gazetteers were forced to accommodate the modules. The result is that if I ever got a campaign to double digit levels I'd have tons of options based on the PCs' prior choices.
Well, that's partially because Mystara hasn't been officially updated since 2e, back when no dwarves anywhere had access to arcane magic. If the situation were reversed, with Mystara being the default 5e world and FR not being updated since 2e, you could say the same thing about dwarves in our hypothetical un-updated FR, since at that point they would never had had access to arcane magic either!