Fountain pens.
Both are writing instruments that are more about a better experience than your standard hardware. I'd consider a ballpoint pen to that of a standard $20 rubber dome keyboard. They both do the same job, but are the bare minimum and can leave people wanting more.
Like switches, fountain pens can come in all different types of nibs (tip of the pen that the ink flows from to paper). From stainless steel to gold that can affect the softness or springiness of the nib. To that of the size (fine, medium, broad, etc) that can affect the size of the line. Additionally there are people out there who will modify (grind) your nib for you to get a better writing experience. I would compare this to people who take apart a keyboard, lube, switch springs, etc.
Material wise they can be made acrylic, wood, stainless steel, brass, etc. There are a lot of different options and seems to be limited to your imagination. People can further customize their pens with various inks. Maybe compare RGB lighting to inks such as Organics Studios Oxygen which sheens in the light
Pricing can get stupid too. Visconti can make $1000 pens, while you can get a $5 starter made by Jinhao on Amazon. There is currently a 100th anniversary pen set by Pilot which I believe is like $30k USD. I'd link it, but my internet is **** on this FOB.
I'm still somewhat new to fountain pens but there are people who make customs as well. Most seem to start with a block of acrylic resin to create a custom barrel and finish the pen with premade components or their own custom work.
I see quite a few users on /r/fountainpens have a mechanical keyboard in the background and started thinking about this, so I was surprised to find an active thread about the topic. Just my thoughts.