It has been long known that the F-16 was in the works. Before the F-18C was announced it was up for debate if we were getting the Viper or the Hornet; up until that point we were only told we were getting a modern Western fighter used by many countries around the world.
It makes sense in the long and short term as a lot of the technology they are building can be moved from the Hornet module to the Viper module with less effort. It will still be a big undertaking and I wouldn't be surprised if some of those steps are already in the works.
I would not expect an F-16 announcement before Christmas '18. That does not mean we are getting one around the time, merely a formal acknowledgement of what is to come. At the earliest I would expect EA to be late Spring of '19.
The refinement of the Hornet will still take a lot of effort over the next 12-18 months after release, though there will be team members that have less and less workload in regards to the Hornet that can move on to the next project.
This is of course, just my merely humble opinion....