Most of this is already said, but there are a couple important things which come to mind. You need to remember to strike the perfect balance of aesthetic and gameplay, making sure there are enough platforms and areas people can explore, loot, and fight on but not making the map feel cramped and claustrophobic. Remember not to add too many chests as there will already be randomly generating ones too, but then again don’t have too little xD
If you’re going to make it look good, never do it at the sacrifice of function. Having a unique theme to the map helps alot, if you ever need inspiration I would recommend taking a look at accepted maps, or checking out beanep’s map list. If you need any help with specific problems, asking on the munchymc discord is a good place to start :D