SSL

I've always been amused by the sound of the SSL buss compressor. And nowadays you can get one in kit form for about $500 USD.

Passive mixing

I've been amused by the idea (idea, mind you, not the reality) of a completely custom-made DIY mixing board.
There is a reason those things don't exist. They're really really really hard to do. The moment you add one new feature to a channel, you may as well just go buy an Allen and Heath mixer and get your work done.
Passive audio mixer w/o pots.
Still, I'm amused by the idea of a 6x2 mixer for classical music. I was kind of thinking in terms of a completely passive design. Feeding the line out of external mic preamps into it, you go through some transformers and Penny&Giles 120mm analog faders, and there ya go.
Still, not quite as easy and one might hope. Actually, what I don't get (yet) is the interaction between channels when you start turning some up and others down in a totally passive mixer.
But in my research I've come upon some amusing things. Like a completely hand-made tube mixer. You're looking at about $1000/channel designed by a former Neve engineer (I know, right?) at Custom Tube Consoles.

Reading up on his designs started me down the path of looking at Helios 69 "passive" EQ designs. Which is kinda cool.
___

Also: 3D printable headphone holders.

Multi-stage saturation "colour" kit.

Wooden mics and suchwise.

FiveFish Audio makes a vacuum tube preamp kit for a 500-series rack for $190.
JLM Audio, in Australia, makes an opto-compressor 500 series kit for just under $400 (USD.)
Here is an early prototypes of the "Wooden Microphone." The inherent structural issue is revealed in the second image.

Oops.

Austin Ribbon Microphone

The Austin Ribbon mic kits are very sweet. All the parts you need and really good instructions. I got mine with the Cinemag transformer. I don't understand why transformers sound so different from one another. That is beyond me. Cutting and installing the ribbon is a surprising level of pain in the tuchus. Lucky for me the Marsian Queen did that part for me. Also lucky? Having a FabLab to build in.

Laser engraved with the secret elephant language of course.