10 Rules for Beginner Mapmakers

4. When creating a map, consider the geographical location and its connection to the plot. If you are making a map in Erathia, take a real map and consider all the features of the continent that were created before you.

Well, you shouldn't. In HoMM3, there is an XL map (I don't remember the exact name, but it's from the original game, not the expansions) where the gods and heroes of Ancient Greece are featured. If NWC thought that it was possible to introduce the epic of Hellas into the world of Heroes for variety, then such an approach definitely has the right to exist.