I'm working with one more quite straight-forward XL-size map for Heroes Of Might And Magic 3 Complete. I'll add it to the updated map-pack of mine once finished. Not sure when it'll be ready, though. It's called "Forest of The Old Godz", and will feature two full-blown "above the ground" maps (underground isn't really underground, at all). It'll be 4 vs. 4 scenario, whereas above-the-ground-layer represents South-half of the overworld, and underground-layer represents North-half of the overworld. North and South halves overlap, being sort of a continuous one big map, and you can reach from North-half to South from South-end teleports (which enter to South-half's North-end, and vice versa), plus one extra two-way teleport in the middle of both halves, connecting them further.
Not sure if the best way to do a map for HOMM III, but when I do, I start with initial very early plan of how many players/teams belong to the scenario, and where they start at. I have an early idea in my head what kind of a landscape to do (to emphasize on forest, desert etc.). I don't think that much of a story (too deeply), since I don't generally play story-driven maps that much myself. If I get a good idea, however, I'll stick with it. I then draw the early sketch of the main terrains with editor and place initial positioning of the castles of each players. It's good to have some sort of an idea of the story of the map, however, at this point, so you will know whether it will affect to the map design, object placement, and later on to events. Then it's time to think how to divide the main-areas of the map (do you make the progress of the map rather straight-forward or more freely explorable), and place the mountains in to the map to see how the main areas would play out, and which of the main-areas can be reach from where. It's a bit like making a labyrinth. Then I just move into making the smaller paths inside the main-areas, add area-related vegetation to each of the main areas, such as trees, dunes. It's good idea to leave fair amount of dead-ends considering small paths, so you can place important well-guarded objects there. When done, it's time to place extra castles, dwellings, and other places of interest in the map. It's good to keep count of how many major things such as level 4 artifacts, dragon cities, and libraries of enlightenment you place and where (to keep things balanced). After this is done, I'll finalize the looks of the map with adding additional terrain/area-related objects and vegetation to spice up the looks of each area. Bare in mind that one-square wide extra-narrow paths may cause some AI issues since only one hero at a time can fit in there (width-wise), and therefore if one hero faces another friendly hero, if the width of the path is indeed one square only, neither one can't pass another. It's good idea regularly to turn on "Passability"-tab in the editor to see quickly if certain narrow paths are impassable or run only one square wide too far. I add monsters and events last, test the map out, re-balance when needed, and sort out errors.
In Forest Of The Old Godz, if you think those two layers, halves of the whole map as one overlapping area, another team starts from the middle of it all (2 allies of team of 4 from South-half's North end, and the rest 2 allies of team of 4 from North-half's South end). You can see that the 4 teams in the middle of the whole map have starting castles rather close to each other, but their connection to each other has been prevented from North to South (and vice versa) and East to West (and vice versa) with Border Guards. So, the middle 4 players can defend each other rather well (sort of a turtle-defense), but only after they have found Keymaster's Tents to unlock their paths to each others.
The other team of 4 players, however, starts in a way that the other 2 players start from very North end of the map, and the other 2 from very South end. They are split so, that only 2 players are close to each other at a time, and have harder time in theory to defend each others, but either stacks of 2 players at North or South don't have Border Guards between them, so 2 North-players and 2 South-players have earlier access to each other's castles in the beginning. Right in the middle of South-overworld-layer is a two-way teleport to North-overworld's middle (and vice versa). I won't make it easy to reach in the beginning of the game, but once you do, you can enter the other half of the map faster. The teleport will make things bit more interesting as you can surprise the other half of the maps players with a surprise-attack. I don't know how well balanced this starting-positioning for players (teams) is, but at least it makes the scenario varying depending of which team you belong into.
I haven't thought of the plot too much, but I can tell you it will be rather shallow. I was thinking of making this map tailored for 2-4 different races. It will be either outer teams being definitely Elves (Rampart), possibly the other 2 being Castle or Fortress not sure yet. Holding a grudge, but against who? That won't be revealed yet. ;-)
Forest Of The Old Godz will definitely be an exploration-map with forest-theme (but won't be just all-around-forest), meaning, that the paths will not run only to one direction in a railroad-y fashion, but there will be crossroads, and the land-area is rather big. That being said, there are some mountain-ranges which do block the most direct ways for opposite teams to reach each others. I will add some tailored events for everyone, but will not go "too deep" into that. I'm not into that far in making the map yet. Perhaps friendly extra troops (small amounts by rather random), some extra encounters fitting the theme with combat and rewards, and perhaps some experience bonuses for discovering certain places.
Here are some early development pictures: