I actually don't have a preconceived plan when I create a map. Usually I am gripped by a random meeting of constructions, which give me a certain impression. This is often an interesting connection due to the placement of passageways.

One of the first things I pay attention to is that each door can invoke a choice of two options for a possible course.
When I have some of these separate parts I try to merge, rotate and fit them higher or lower.
When this yields something, I try to make the atmosphere more grim or lighter for lighting.

At this point it is better to leave everything for a while. The direct involvement often makes mapping too exhausting.

I am not good at placing enemies, it is often enemy behind enemy. What I can't stand is not enough health, putting enemies behind my back, a trap that ends the game unnecessarily early.

But to be honest, these things only happen in the light of enthusiasm. It is not something I think about. Some things seem beautiful, but are like a house of cards, the higher you go the more likely it is to collapse due to previously made bad constructions or a dead end of a leak, which you don't want to lose. 

Always depends on what i want to achieve. Do i want to troll the player? Do i want them to have a good time - do i want to teach something?

Also strongly effected by the game i am designing for. Can I goof off and do something crazy for a private project or do i have to be stupidly on point down to every last f'n pipe.

In the end i use concepts/goals to incorporate into and throughout the level: up is good, down is bad, stay healthy, don't use all the ammo, be scared of key elements, you can expect ambushes.

But then, sometimes just break expectation and down is now a good thing. Put a secret there. Or as in your case, if you had the player run through a pipe level, suddenly make them choose between 2-3 directions.

I feel like surprise can keep a level going. - like giving the player a super powerful item, but nothing happens. Anti-climactic sure. But it will create a weird sort of tension for the next couple of minutes.

Anyway thats my take. 

I visualize in my head what it is I want to create, I decide on a theme and draw a picture in my mind of what it could look like. I'll spend time scouring the internet of photos related to the theme to give me inspiration, providing an even clearer visual of the map.

I always like to use my own (not necessarily owned by me) textures in my maps, so before I start creating any of the map I search for textures that I feel would be appropriate. I'll use texture website resources, search through catalogs, regular photographs of things and textures from other games. I plug them all into photoshop and begin the process. Depending on the picture sometimes it takes only a few minutes, and on the rare occasion I can spend an hour getting a texture to look just right. I'll create tons of textures to make sure I have more than enough, and almost every time I'll wind up never using some of them.

I don't ever have a plan of how the layout of a map will be, just how the map in general will look. I'll start mapping and ideas come to me as I go along. Interconnected paths are important, so oftentimes I will deliberately leave a space in a room totally blank in the event that I can figure out later on how to connect it with a part of the level in the future.

I do the heavy detailing and lighting as I go along building the map, I don't create the basic layout first then detail later, so once I build an area its nine times out of ten the final product. Not the professional way to do it, but it works for me. One of the things I strive for is making each room visually different, putting in landmarks to keep the player from getting lost, knowing where they are at all times.

Once the layout, details, and lighting are finished to my liking, that's when I start doing enemy and item placements. I set enemies based on where I think they'd be waiting, I try to keep it diverse as I can so as not to bore the player with the same enemy over and over. Most item placements such as health and ammunition are in the corners of rooms, whereas things like armor and power ups I make sure are out in the open, visible to the player, if they aren't in a secret area that is. Speaking of secret areas, I keep them to a minimum or don't make them that hard to find. Only because I'm terrible at finding secrets myself.

That's pretty much my entire process of creating a map from start to finish. 

Each time I try and do a different method to narrow down what are the most effective.

Loops
These are your friends - make the player go back to where they already were, but from a different direction or by changing the environment enough that it feels new. If you get it right, just the act of retreading the same route backwards feels fresh.

They're also the Quake map ideal, established by id. See key door, find key, arrive back.

But...

Your question appears to be about gameplay rather than the technicalities of structure.

I like to use all the tools. Players like Quads. Players like high tier weapons. Players like slaughtering hordes of low level enemies in varying situations. Players like frantically surviving a Shambler ambush with the axe with 1 HP left.

If I feel an area is boring, I add a Quad and (at least) a Shambler.

If it feels like the player is spending a lot of time when a specific weapon or powerup could let them do the same but either faster, or in a way that makes them feel clever - give it to them. Not in some convoluted secret, just plonk it there in the middle of the arena or whatever.

Side note on secrets: They only have two functions. Make the mapper or the player feel clever. When well thought out, they can achieve both, but that's rare.

Never lose sight of the fact that the game is supposed to be fun to play.

Powerlevel can work, but it gets tedious fast. It's why Telefragged, which has a very broad powerlevel (the player can't even attack at the start) has so many Quads in it towards the end. Otherwise it would have been boring.

As I said, I like to use all tools in the box. If you want open, explorable levels, make the player find five silver keys in order to complete the critical path, in any order they feel like. Just make sure to show them the SK door before they start that quest, and, ideally, the silver keys just out of reach various times as they loop through the level. 

In order to be successful, I usually need to have a core idea of what the map is supposed to be.

Take sm207_ionous for instance. The idea was Februus Depth, but using ID1 textures and speedmapped.

I built one room to test the idea, and found that the brushwork I was generating matched the idea, and worked on an aesthetic level. If I can that part down, I'll just build until an actual level begins to emerge.

Gameplay is last. I can move around architecture and make new pathways to accommodate any of my gameplay notions. 

https://mobile.twitter.com/the_Norberg/status/1269341626951925765

I wish I was so orderly when planning.