An interesting alternative would be a “synth” that essentially loads VSTs, in which case there are lots of good Wavetable software synths. But that project would come with a whole range of headaches and I understand why it hasn’t been done. Or at least, hasn’t been done with any real success. But man I would love to have an Omnisphere 2 dedicated synth.
Actually a few of companies have made VST players for computer-free softsynth performances. There is SM Pro Audio's V-Machine and though the product is still available in many places, I don't know what's happening with SM Pro.
Another is the Muse Research Receptor which comes from a more stable company. Still only a two line display and four knobs.
Then there is the Seelake AudioStation X64 which looks like the best solution of the three. It has a knobbox like the old Akai samplers from around the turn of the century and that is a blessing. Though it still only has a two line display this can be complimented by a tablet.
I've been tempted to get a VST player, but things like compatibility of VST's and it being basically an exercise in old fashioned menu diving has made me think twice. Last I looked the AudioStation X64 wasn't out and it is quite an impressive computer and probably has a price tag that reflects that.
As far as being devices for complex programming of soft-synths, I don't think they would provide a too satisfying experience, but would be great for stable on-stage VST use. I could be wrong about that with the AudioStation X64 and it's tablet support, but then you are bringing a third-party device (compatibility issues?) into the system and those things are little VST hosts themselves in a way, though they don't have anywhere near as decent IN/OUT options as the dedicated boxes above. It seems with soft-synths is always a trade-off when trying to make them as flexible as stand-alone hardware.