Only Apple knows the answer, but most rumours point to some sort of special "10th Anniversary" model next year, breaking from the traditional 's' cycle.
Very doubtful there will be a 7s. The next iPhone will need to have a dramatic redesign. There won't be many Apple fans tolerating a fourth series of the iPhone 6 design (iPhone 6 - series 1, iPhone 6s - series 2, iPhone 7 - series 3). If you listen closely to the iPhone 7 design video, it concludes with Jony Ive saying something really interesting: "When complete, iPhone 7 is the most singular, the most evolved representation of this design." This hints that it is the end of the road for the current iPhone design.
Very doubtful there will be a 7s. The next iPhone will need to have a dramatic redesign. There won't be many Apple fans tolerating a fourth series of the iPhone 6 design (iPhone 6 - series 1, iPhone 6s - series 2, iPhone 7 - series 3). If you listen closely to the iPhone 7 design video, it concludes with Jony Ive saying something really interesting: "When complete, iPhone 7 is the most singular, the most evolved representation of this design." This hints that it is the end of the road for the current iPhone design.
I dont see how they could release an 'S' as I dont see how they could enhance the current model which is normally what the 'S' is. The 10th anniversary phone may be released in June to celebrate 10 years and will be completely new redesign if the rumours are true. Maybe they will call it the iPhone Decade because if it is a radically new phone then just calling it an '8' may do it an injustice.