T78: There are certainly basic precepts that are both normally followed, and generally are wise to follow for good measure. According to the guidelines in the map-maker tutorial files that come along with the mapmaker, and those found on the official sit where you get it from, your maps should have at least two approaches to the main base, to permit flanking attacks, as is seen on the Eriadu base game map (and most others in vanilla. They do follow their own rules), seven base build slots, even if the ground base isn't intended to have that many ground build slots, so you can move your buildings around. It requires some perfectly open space to serve as reinforcement points- failure to properly accommodate these with flat terrain can lead to ground units getting stuck on when you call them in. There's a lot of basic stuff you can find if you google "sweaw map editor tutorial."
That said, there's no single "perfect" map. I'm doing my best to create interesting and rewarding maps to play that work, look good, and support strong decisions- in exactly that order. Placing a base and an attacker entry point are first- this lets you see the avenues units will use to either attack or defend. Then place other structures and buildables and such to suit the terrain. A mix of open and tight space offers tactical opportunities, so I'm aiming to mesh both, with larger open areas and tighter city streets and ravines to traverse. An excessively open map makes it harder to find enemy units as there's so much space to get lost in- too little open space and they won't be able to move effectively, or in extreme cases, get stuck. So there's something of a balance to strive for with terrain open-ness.
But before that, you have to make sure the map works and the placement of things makes sense. You may not appreciate how much you can change a map by moving one structure on it. Example: The buildpad guarding the open water on aforementioned Eriadu. Take that away and waterborne attack are extremely difficult to repel, even though you didn't add more water. On the same map: move the base out of the city, and the defender won't feel like they're under so much early pressure. Any change affects how the map plays- a lot of maps that are annoying just need a few tweaks to be made far more satisfying to play. Bunker pads at the end of dead-ends moved to choke points, reinforcement points that are currently broken can be fixed with little effort once detected. It's really these things that decide how the maps are to play, not how open they are, in a lot of cases (there's one in the mod that's FAR too open; a desert map with a single assault avenue to the map's east, and all four reinforcement points in a huge open plain. I dislike this map for being too open, but it's an extreme case. It could be absolutely remedied by the addition of one, maybe two, abandoned sensor platforms. Seriously; that one change and the problem would just melt away.
I hope this helps, mate. Please, hit me up if you have any more questions. The more people know about the mapmaker the more likely some other folks will start mapping, which would make me happy. Also, a huge long shot but I don't want to pester Corey again, so: do you have any idea how populous Pantolomin (Panto Prime) is? I have trawled every lore source I know about and can't find much of any lore, let alone a query so specific. But, in this place I'm surrounded by people who know even more about Star Wars than I do, so maybe I'll get lucky
Just want to know how densely to scatter civilian spawners.
Tom, Corey- sorry to interrupt,
and fits into your overall concept. I hope that I can streamline the process in the future to approach mapping a bit more effectively, so I won't spend half a decade creating a single one... thank the force xD for Wookiepedia though, it actually helps a lot evaluating what could fit and what not. 
, happy mapping!