/// Tips on Height Maps \\\
If your map is going to be mostly land make sure you use a dark grey for the base of your map. That way you can build height a lot easier and a lot more interestingly.
Your height map does not need to look perfect! It could be a mess of smudges and oddities, it just gives you more to work with in design! It's more interesting, too.
/// Tips on Terrain Maps \\\
Use rough brushes for paths and patches! The softness of the brush all depends on the environment, like if there was a lot of snow. I'd use a messy soft brush to mark out my paths.
Try to use all three possible terrain differences if you can! RGB! Different grounds add great detail to your map, more than you'd expect.
If you're using a light terrain texture (like snow), some heights can be difficult to see. Use a darker or slightly darker terrain texture to refine the 'dips'. It'll look like a cast shadow almost.
/// Tips on object placing \\\
Trees are more interesting when they come in different shapes and sizes, but this doesn't necessarily mean you need to be dramatic about the differences.
When placing bushes, don't just place one, place multiple and create a cluster of them. Not all have to be completely above ground either.
Always turn repetitive objects (like trees, bushes, and other foilage) so things don't look absolutely copy & pasted.
Trees also make great bushes! The acaicia tree for example, sink the trunk beneath the ground, and have the branches growing out. This works for other trees too!
Not all objects need collisions- only add/keep collisions on objects you KNOW people are likely to use! This will reduce so much lag!
Everyone loves to take screenshots in maps! Make sure after you've placed a few objects that you make sure it looks aesthetic! This works better in test when you can see a character!
Don't make objects too big or too small! Be realistic! Unless the whole theme of the map is big or small...
I'll try to think of more if I can