I'm personally fond of this one though: https://en.wikipedia.org/wiki/Moog_Taurus
See also this classic instrument, familiar from the late great FAO Schwartz on Fifth Avenue: https://en.wikipedia.org/wiki/Walking_Piano
With a foot activated MIDI controller the number of keys will always be rather limited. Therefore, I think that the virtual layout should be customized to the music you play or even for each song. E.g. you could only map notes of a certain scale and range in order to save space.
If you consider building something custom; I have found that outputting to hardware MIDI from an Arduino is delightfully simple. (The hard part on my own project is finding motivation for figuring out how to read velocity from piezos and isolating vibrations.)
The number of keys is the real problem - I don't want to be playing a single note in one octave, I want to play my chord progression and that means I need separate buttons for the major, minor, 7th (which 7th...) for the chords that go with the song.
Are you looking for a mode which allows you to select a chord rather than just a single note? (One foot for the root, and the other selects single/major/minor/etc)?