The MIDI Melody Maker from adafruit.
This is a kit that uses the Cortex M4 and gives you an inexpensive versatile base of hardware to code your own powerful custom MIDI processor. ( Can be USB or UART, AKA DIN MIDI based. ) It has a nice small 128x64 OLED screen, and a range of controls and UI that would be easy to modify. There is an open source basic system, but it just calls out for customization. The base code is in easy to program CircuitPython.
You like the idea of the SOMA Metaconformer ( thread ), you can write your own version.
Or you have an idea inspired by your use of the Blokas Midihub ( thread ), that’s almost exactly what you want except for the last few things, then here you go.
Your own arpeggiator, chord generator, MIDI LFO machine, whatever !
Or you are exploring another idea that you think might make an awesome product, and you want to do a prototype to prove things out.
You can see more detail and order from this site :
I am not thrilled about the enclosure, but it would be easy enough to do a much more compact and rugged box for this.