Inbox by Gmail was intended to be the new version of Gmail, and they decided to make it a separate site after the internal backlash against the Gmail team by other Google employees for "ruining" Gmail.
If you want the newest, Googliest version of Gmail today, use Inbox. And then there's Gmail, for those who want a dependable, stable email client that has all the features that people have come to depend on over the past decade.
But out of curiosity, what would you like to see in gmail as an update ?
For example, Yesware, Hubspot For Sales, and ToutApp are all built on top of GMail probably using the gmail.js library - and I'm sure there are a lot more.
I'm not sure they have the clout to keep Google from changing, but it's something for them to consider.