All the Worlds in the CD&D Reality:
CD&D Dragonlance
CD&D Forgotten Realms
CD&D Eberron
CD&D Dark Sun
CD&D Nerath
etc.
A helpful exercise is to imagine if the BECMI line continued to sell as well as AD&D, and so continued being produced up to the present day. And so various BECMI gazetteers or sourcebooks were released for these other settings. Or the key books (FRCS, Living Greyhawk Gazetteer, etc.) were released with dual-stats.
It's fun to imagine which features of the Mystara setting are depictions of the "BECMI Rules Reality", and which are features of the World of Mystara which would be depicted in any Rules Translation. For this exercise, we ignore what 2e Mystara "should've been" like, and stick to the conversion method the TSR designers actually used in the published products. Closely comparing the 2e stats of Mystaran NPCs to their BECMI stats is one example. It could be argued that Mystara 2e should've made use of the higher-level Player's Option rules which support characters up to 40th level, since CD&D went up to 36th level. However, in the actual 2e Mystara products, the NPC levels are collapsed into the 20 levels of 2e.
So for this exercise of imagining "lost TSR BECMI worldbooks" we do the same. Then the 20 levels in a Dark Sun campaign are expanded out to 36 BECMI Dark Sun levels.
Once we've reached clarity about what is CD&D Rules Reality, and what is Mystara Setting, then reverse engineer the CD&D Rules Reality to the other settings. Is L-N-C alignment essential to CD&D or not? Or would an officially published CD&D Krynn use a G-N-E alignment? etc.
All the Worlds in the SAGA System Reality:
See that Dragon magazine article which addressed this.
All the Worlds in the First Edition Reality:
What would 1e Mystara, Eberron, and Nerath look like?