...I can imagine DSI staff sitting there, laughing at all this guessing about
It's probably the only joy they get in life, so happy to help. Here we go...
Actually, a joint venture isn't too unlikely. Tom Oberheim helped out already on the state variable filter of the Pro 2. And in a recent Q+A session Dave mentioned that the next product would be a surprise.
We're assuming that this is a picture from a part of the new product. Maybe it's not. We know (esp. from the Tempest) that DSI likes a bit of wordplay in their pre-announcement clues. "Portamento" is a near-homophone for "portmanteau," which refers to a word that arises from combining two previously-existing words (e.g., paratrooper and... um... fleshlight). So the new instrument could be a hybrid of some type, either architecturally, or as a joint or cooperative venture.
To take this ridiculousness ever further, a portmanteau was also a suitcase with two large compartments. So perhaps an instrument built into its own case (think ARP 2600).