The keyboard/module, and 'size of keyboard' conversation is very interesting. There has definitely been an evolution away from linear 88-key controllers towards smaller keybeds, and alternate devices for midi input.
I think there are basically two current trends: One is about portability, the other one about alternative input devices that are able to offer a more expressive articulation. While the former one is mostly based on sheer practical concerns by touring musicians or those who perform live frequently, the latter one is about more fundamental questions of musical expressivity, especially since the keyboard (based on the organ, not the piano) is a rather primitive on/off switch device.
A new DSI synth would have to offer a compelling set of controller features for me to invest in anything but a module.
With regard to alternative input devices and controllers, a welcomed feature for not only a brandnew instrument would certainly be MPE support.
I believe many of us who are willing/able to invest in boutique synths already have a number of master controllers and alternate input devices. I don't have space, or need in my studio for another full-sized keyboard. At some point I will get a Roli-like device to explore the additional dimensions, but I think that market has not yet reached a sweet spot.
That's a valid point. With a certain amount of gear already present, one would not need more and more keyboards along with every new synth engine. However, people also tend to sell stuff in order to buy new instruments. So there's not necessarily a growing number of keyboards in every case; not everyone can afford to be a collector, and not everybody wants a plethora of synths, but rather a limited selection of reliable tools (depending on personal goals), of which one can only use so many in depth anyway. And then there are those for whom a new synth is only a true instrument if it is a self-contained unit with an appropriate input device/keyboard, which also makes sense, especially if you don't wanna carry around lots of different devices all the time, which could as well be included in one box.
I'm not sure about whether the market has not yet reached a sweet spot yet. There are as many alternative controllers around like never before and one could even make a joke about how many new kickstarter campaigns for whatever kind of controller have been started this week. There's definitely an extreme differentiation going on with more and more specialized controllers that come in all variations - from building blocks to ROLI-like approaches and even apps. I would agree with you insofar, as some projects appear really forced, or just as a slight variation of another product. All the rest might be tied to the question, whether there is anything like a perfect controller or input device. I'd answer that question with 'no', since each one of us has different goals and techniques in mind. So the whole controller market is one for highly individualized needs. Some controllers offer different modes of operation in order to cover different approaches, like the Linnstrument for example, others are more specified. Then, it's also still important how a specific controller feels to a performer. A ROLI for example might just not be for everyone.
The biggest problem I see though, is that of adaption: Not everybody is willing to un- or re-learn a playing technique just for the sake of a different controller. Many people still feel comfortable with a keyboard, no matter how limited it might be at the touch of one button. If someone also plays an instrument with a fretboard, the transition towards a Linnstrument might not be that hard at all. That's basically why I think that alternative controllers won't become a mass market phenomenon, at least not in the near future.
At this point I'm just not sure I need anything else! Between what's available as synth software in the box (Macbook Pro), a new external hardware synth will have to provide a sound I just can't get from my P12. So far - I haven't hit that wall.
Well, generally speaking I think that the next DSI instrument would have to be substantially different enough from the Prophet-6, OB-6, Prophet 12 and Pro 2, yes.