I've been on the quest of a universal hardware surface for all my soft synths for years. The attraction of hardware control for me was/is the ability to train muscle memory.
However, I've come to the conclusion, that there's no hope in hell to have a consistent hardware interface for a very wide variety of synths from different manufacturers and with widely different functionality. A wall of 500 knobs and buttons wouldn't be enough for Omnisphere alone.
And since most soft synths these days have entire FX racks built-in, that really deserves mapping, too. All of that gets out of hand in a real hurry. I got to 32 controls in no time for just the most important parameters of a relatively simple MSoundFactory sampler synth with a few FX (chorus, delay, reverb) I built.
To get there you'd have to make or buy into somebody else's own mental model overlaying all of those synths, mapping the myriads of controls of each synth to a simplified common subset of your liking and then map the hardware of your choice to that. Volume ADSR, HPF and LPF and resonance are the easy one's but after that it gets messy very quickly - even more so, since many soft synths have multiple layers.
Also: wavetable synths benefit from different controls than subtractive or FM or granular etc. Even samplers vary widely. And I already mentioned the FX problem.
Every once in a while I've started to work on creating a giant map of this nature, but then I get distracted by playing and/or recording with all of those wonderful soft synths and fx. ¯\_(ツ)_/¯
p.s. As an exercice for the reader: Just try to pick 32 paramaters you'd map in common for the top 5 soft synths in your rotation. If you think it's easy, post it here.