I enjoy maps that are immersive-- whether that is through a very unique and complex landscape or a simple one, as long as it makes me feel like I'm in the "world" that it exists in, it has a special place in my heart. A great tool for immersion is to focus on detail and bringing out elements in the environment to cultivate an atmosphere-- the specific details that Sophy mentioned regarding toxic clouds and green goo are excellent examples of little aspects that would formulate a specific environment; they're thoughtful additions that display the creator's attention to their creation and will stand out. In addition to the minute, the overall/big picture of the map is also important, as this is what most people will see before looking closer at details and will generally spark the first interest. My advice would be to play around with what you're interested in, though, and don't become too focused on separating the big picture from the small details, because often those qualities will overlap. Think of a place you would enjoy to spend time in. ^^
In regards to specific environments, I can appreciate any map as long as it is committed to the environment that is being communicated. I personally love cold/winter maps-- I think it's a character thing. I love winter and have created a few characters centered around mountain landscapes. I think a space map would be cool-- it's a bit ambitious since the available outer space meshes are few, from what I've seen. With a map concept that doesn't have as much foot room for object variety, this could affect how detailed you're able to fashion the map (so, that is a drawback from a unique idea. Advantages and disadvantages to all themes. c:) A good forest map can go a long way, too. One tip is to vary the sizes and types of trees and make sure that there aren't too many gaps. Someone mentioned how applying many meshes in one map can cause lag, which can indeed be a problem, so I would suggest using objects with low poly, while still maintaining as decent quality as you can.
One last recommendation is to look at every "piece" of the map as being essential. Sometimes, it can help to divide little sections on the heightmap into specific areas that you have in mind. (Example: If you're going with a simple meadow map, you may have a section dedicated to a waterfall, a section dedicated to a large purple flower patch, and another section dedicated to a den site.) This will help to reduce any wasted space or areas where you feel are just a bit "there" without serving a purpose to the map or enhancing it. Many map makers don't use that setup and will create as they go or mix things up, so it doesn't determine how good a map is, but I've found that it can aid you as the maker and others in distinguishing landmarks for navigation and cleanliness.
Good luck with your project! It's a very thoughtful concept to include the community in the making of your map, and I'm sure it will turn out wonderfully.
