Keep it interesting and most of all fun, try bring something new to the table which may not have been used in other maps. For example, is there a certain video game that you really like playing which you could make a map based off which nobody or not many have done before?
In terms of gameplay keep it interactive with the player, depending on the map and intended gamemode the map will be layed out differently, a deathrun map being more linear versus maybe a versus saxton hale map being a bit more open and additional hiding spots and diversion routes. Hammer has many functions where you can do all sorts with the player whether you want to fling them around, damage, parent entities like trails shit this isn't even a fraction of the stuff you can do. I would recommend watching Top Hatt Waffle his videos have helped me greatly.
However there are waaaaaaay more experienced mappers here that will probably have a better answer than myself