Analog Keys MkII seems like a really obvious move.
If they go for AKmkII then some sort of a digital keyboard might be in the realms of possibility. Think of something like the sampler keyboards from 80s but modernized. Digitakt already has pretty great possibilities for melodic synth stuff and pads and they could explore the full potential in this format. Less tracks but more polyphony, perhaps more focus on multisampling and so on. This would also cater to the experimental synthesist crowd.
Another possible device could be something like “Analog One”. Think of a Digitakt-sized single-voice Analog Four. If Electron would want to aim for the two-osc bass box market but with their unique twist then this could be a very legit business decision. They could perhaps even recycle the Digitakt chassis for this.
What I’d really want though would be a dedicated Elektron sequencer with competent song mode along all the usual bells and whistles.