For the longest time, "ECMAScript 6" or "ES6" were the official terms used to refer to the language version succeeding ES5. Over the years, we've had ES1, ES2, ES3, ES4 (which never came about), and ES5. Hence, naming the new version "ES6" would only be logical.
Shortly before finishing the work on ES6, however, TC39 decided to transition the ECMAScript standardization process to a yearly release cadence. Each new release would carry the standardization year in its name. Thus, "ECMAScript 6" officially became "ECMAScript 2015".
Here's the thing, though: "ES7" is nowhere near as widespread a term as "ES6" was or still is. Therefore, I suggest we drop the term "ES7" in favor of "ES2016" — once and for all. Let's call the child by its name instead of coining an unofficial second term.
Naming is hard already. Let's not make it even harder.