They’ve kind of hinted at what they were going to do with the naming convention this year. They skipped the 7s and went straight to 8. My guess is that in 2018, there will be a 9 (really 8s) with some incremental updates. There will also be the XI or 11 that will introduce the Plus version with updates and possibly another color added.
In 2019, full steam ahead with the XII or 12. There won’t be a new model of the 6/7/8/9 form factor. They will still be available in the iterations Apple sees worth continuing to offer for lower price entry, but that generation of design will be done for seeing updates.