For one brief shining moment, Google Inbox existed. It was the best mail client I've used, dating back to Eudora. Simple, focused. Not everybody liked it, but it was one of the few apps that I immediately loved, and that made me more productive.

Then Google sunsetted it, and folded some of the best features from it into Gmail.

Heads up: you don't fix an overly-complex application by adding the features from a very simple application to it.