Where to build a Midi-Controller prototype?

[jam-s]

You will need an electronics circuit and pcb designer, a mechanical CAD guy, a firmware coder and quite some cash to pay these people for their work (depending on the complexity of the project a few hundred hours each). Then you will need somebody to assemble and debug everything.
There are also companies that can offer you these services, but it will most likely be even more expensive.

[gullum]

based on the amount of replays I would say no. nobody has any details

[Sterioevo]

https://yaeltex.com/en/ I cannot vouch for them though, never used any of their products.
You can always build a prototype (with scripting) in lemur or touchosc type iOS apps.
And perhaps the most direct route is to write custom remote codecs for existing midi controllers, for example it is possible to create “components” on a controller as different virtual controllers so the knobs could be attached to combinators, and faders to the mixer, etc. with 2 way feedback of values. Look up koshdukai if this tickles your interest.