With regard to the Mystara world of Basic D&D, there can be glaring conflicts between the fluff of one text and the fluff of an other text.
For example, one of the Basic premises of Basic D&D is there are "no gods". Instead, player characters can become powerful beings called "immortals". Yet, Mystara rife with "gods", including culturally insensitive misrepresentations of the Norse aesir as if gods, and having little to do with reallife Norse textual traditions describing these aesir nature beings.
In any case, player characters can become cosmically powerful immortals − at epic tier − without being "gods". This is a salient aspect that makes Mystara setting distinctive. A 5e version can focus on this, and either recharacterize or remove the parts of Mystara that contradict this central theme.