One already knows the range will mimic the Panamera range, Turbo with 550HP and the Turbo S hybrid will have the e-motor and 680HP.
Size wise I don't see it getting any bigger significantly than the current version.
Big news will be the touch button centre console and the mostly digital dash with analog Reve counter.
Unibody will likely be lighter than the current generation, how much I don't have exact figure yet.
Biggest news from Porsche could be reserved for Geneva 2018, we already got notification from them that they are planning an 'exclusive' event for the 918 owners. Expect something limited in numbers.
--