Creating a good map for the old classic Doom can actually be quite hard. Using modern wad editors is fairly easy, but creating new levels, that will also be worth playing for old players, can actually be quite challenging. If you take a look at various wad files available online, then it quickly becomes clear that most of them suffer from various balance issues and smaller bugs. Some were even created for a specific modern engine without clearly stating which.
To create better levels, there is some fundamental design ideas that one can consider. For example, moddeling a map based on real-life houses will often end up boring to play. Simply because the doom engine is not ideal for this type of layout. You can fake it by creating and shaping rooms into objects, but it generally looks bad, and the map can end up with a "static" feel to it.
Another mistake to make, is to aim for too much detail in the levels. The result is the levels get a "cluttered" feel to them, again because of limitations in the engine. Not only this. The levels will also take longer to make, often end up as poorly balanced (difficulty), and they might crash or perform bad on week computers.
Good level design respects engine limitations
When you look at some of the most complex levels – often including roads, cars, and caves – These maps can be boring to play, because the obvious limitations of the engine distracts from the gameplay. In other words. Avoid creating roads unless you are prepared to create real 3d modeled, derivable cars. Stick to the original level design techniques.. Do not create large open areas with static content in them (like cars).
Duke Nukem was very innovative for its time, but it was a mistake to include cars in the levels, because people get disappointed when they can not drive the cars. The idea should be, not to include something that the player can not interact with in some way. There can be exceptions to this rule – the important thing is you think about what you are doing.
Improving your level design skills
Doom editing is a good way to learn level editing, and develop your own editing style. Playing through maps created by other people can also be a good way to find inspiration for your own map making. Do not just re-create something that has already been done a million times. Create your own style.
You can even create your own textures. For this, it is recommended both to create a low-resolution version of your textures, and a high-resolution version to be used with new engines.
If you include new things, such as room-above-rooms or water in your maps, be sure that the engine you are using actually implements it in a decent way, an beware that your map will not be playable in all engines.
Backwards compatibility with classic Doom is not really necessary, since people generally use newer engines. But, it can help you stay within certain reasonable limits. For instance, larger is not necessarily better when it comes to Doom level design. A tendency some people have, myself included, is to start out with the aim to create a huge map. It is much better to set out to create a good map, and roughly stop working on it when nothing more can be added to it, and done balance testing on different difficulty settings.
Always add support for multiplayer. It is a huge fun to many players. Cooperative mode is especially fun in Doom, so you may want to design your levels with this in mind.