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.
Also: 3D printable headphone holders.
Multi-stage saturation "colour" kit.