The thinking is that this new platform will be shared by the 911 and the 718s – Boxster and Cayman – able to accommodate the 911's rear-engine layout as well as the less-expensive cars' midship powertrain location. This will require a good deal of flexibility, as the 911 offers a back seat while the mid-engine 718s do not. There is also a strong possibility that this platform would be shared outside Porsche to other VW Group brands; it's the kind of cost-saving move VW really likes these days. There are rumors that the modular platform would be able to underpin successors to the current Lamborghini Huracán and Audi R8, for instance.
The development mule in these photos makes use of a 991 body with some slight modifications. It appears to use GT3 fenders front and rear, with with tape covering the vents behind the front wheels. A subtle extension of the rear fenders could point to a wider track overall for new models, or at least the sporty and all-wheel-drive variants. It's hard to glean much else from the shots, although we do note that the wheelbase doesn't seem to have grown any. That, and a 911 looks pretty weird without a Porsche crest on the nose.
We don't yet know what chassis designation the upcoming 911 will carry, but it would be logical to follow the current 991 with a 992. That said, Porsche platform naming often defies logic – the 991 came after the 997, for instance.
Don't count the 991 out yet, though. The current 911 still has a lot of life left in it, with new versions still in the pipeline.