How do you guys come up with ideas for your maps?

Normally, I watch TF2 gameplay, which allows me to look at the maps without the distraction of being shot at.
This tends to get me excited about making something that someone would want to play, and looking at the maps makes me think about what makes it fun, which leads me to want to make a fun map that uses similar fun mechanics.
On top of that, since I'm quite early in the mapping journey, I frequently realise new things about TF2's design that good maps support and bad maps don't, and that makes me want to surpass my earlier maps by making a new map that incorporates these new design principles.
As a side note, lurking in voice chat and watching imps satisfies both of these, because I get to watch TF2 gameplay and hear people think/give feedback on design elements.

On top of this, I find that I have a 'store' of cool ideas - I find that inspiration strikes randomly, with some small dependence on the factors I talked about above, so I amass new ideas at a fairly decent rate - I then evaluate these ideas, and if they're too few in number and/or not good enough to make into a full map, I 'store them away'. This tends to leave me with new knowledge that I can make fun mechanics out of later, and with knowledge you gain even later, you can sometimes make good mechanics out of your stored mechanics.

For instance, in the map I released half an hour ago, I had wanted for a long time to make "SD but it's good" - you could call that a 'stored cool idea'. Previous analysis of KOTH and present analysis of class balance and choice led me to make mechanics which suited the gameplay of that map and turned it into "KOTH with extra steps", thus turning stored cool ideas and knowledge into a map.

Usually, I look at existing community and official maps, and decide, "I want more of this". "This" could be an aesthetic, a gamemode, or layout/approach to map design. Then I make that.
For example, I'm working on a PD map right now because I greatly enjoy the Player Destruction gamemode and would like more good PD maps to exist. It's also going to be in the midwest farmland (2Fort, Granary, Bagel) aesthetic because, given that it's one of the more fundamental settings for TF2's artstyle, it's gone severely underused imo.
A good facet of this approach is that If you ever get stuck in terms of map layout or aesthetic, there's some sort of a pool of references you can use to help you think of ideas.

That, or I was messing around with map logic and stumbled onto something potentially interesting.

In TF2, I tend to build up knowledge of the game, how Source works, etc. That creates the foundation of what I want to build onto. A good game-sense is key for any development of a map.

Outside the game, and before I even start Hammer, I tend to let inspiration take ahold of me before I start any mapping. It starts with an idea. This can strike any time of day, so I tend to keep something nearby to write down my ideas. For me it's when I'm most "In the zone" (which varies where I am in life).

I have an uncomfortable fascination with experimental game-modes. Do note that every artist is allowed to take things from other, greater artists!

I've made a L4D-style game-mode based off MvM, added different buildings to the Engineer in Medieval, and some other offbeat stuff. This all started with a fascination and curiosity for what I'm interested in, I guess.

I try to do stuff that hasn't been done before = why people also don't play my maps on a regular basis.

i.e. Lost Horizon - was multiple payloads where blue pushes all 3 carts at the same time

I only mess around Hammer just for fun and I have made 2 maps so far: ctf_cabin and koth_green_hill. (ias well as SFM maps forest_clearing and lift but they're mostly scenery.)

I made cabin when I was starting out and it's just awful, don't bother with it. It's painfully beginners work. I have been working on a completely revised version of Cabin but I haven't touched it in a while. My inspiration at the time was Christmas and I figured "If Christmas is about giving, what about a "Deliver the Flag" map?" Problem is I made a mess. But I might redo it from the ground up someday.

Green Hill was an attempt to make a Sonic themed map. There's plenty of various Mario maps out there but no Sonic maps, so I tried to tip the scales so to speak. The map is rather amaturish in design but it still plays as a small-medium sized KotH map.

While I am currently making another Sonic-themed map I plan on making a TF2-standard variation of it as well, 'cus I know not everyone has the same tastes as me.

I make the layouts for my maps as I go, using a small, simple idea for lore, aesthetics, or gameplay quirks as the basis.

Such ideas from my recent maps have been

>what if it was an applejuice factory where you could follow the route of apples being pressed into juice?
>what if there was an area that looked like that one part from Firewatch?
>what if you had to jump off a steep cliff into the ocean to capture a flag?
>what if the payload cart was mounted onto tram tracks that run trough a city?