My comment might be unhelpful, but in the end I settled on using the computer keyboard and focusing on programming the music(via macros, scripts, etc) vs performing it. Because it's always there, there's nothing additional to plug in. Apart from that I have tried a number of weird things:
I have a toy Casio to bang on if I want to play keys, though that admittedly isn't MIDI(Yamaha's PSS-A50 is, though!)
From time to time I've considered getting a Critter & Guitari Organelle: also a complete instrument, with small keys in an interesting form factor. It's open enough and programmable enough to serve nearly any synth need.
I've considered repurposing ortholinear mechanical keyboards. It's mostly an issue of sheer DIY effort. There are some examples of putting velocity on standard layout keyboards...
I have not one but two exotic isomorphic MIDI layouts: the C-Thru Axis49(relatively speaking, portable enough to cram into a backpack, but a bit thick, fragile when dropped, and the manufacturer is long gone), and the Chromatone CT-319(full-sized, really nice build, cheesy built in sounds, manufacturer cleared stock for shipping price only). These layouts are wonderful for hands-on play with scale and harmony ideas, and can be experienced more cheaply with touchscreen apps, though I've never found an app in this category that I'm totally pleased with.
It's actually the isomorphic boards that got me to think in terms of programming it all in the first place. If you reduce the keys to the ones you need to play the particular track you have, you only need maybe 16-24 most of the time(7-8 note scales across 2-3 octaves). And you can easily go into a wholly rhythmic approach by being sample-focused, and then just use drum pads to perform.
There's a lot of options once you break away from resembling a piano. None are exactly better because a majority of people would rather transfer piano skills and play piano music, than play a new thing. That's just how it goes with music.