There was a total revamp to GMail called "Inbox" that Google was working on for a while, but it got deprecated and cancelled before it was released. From what I heard, they were testing it internally and most people liked it. It just was more search focused so the backend costs were more than they thought they could bear and it was scrapped.

I loved Inbox, it was way better at organizing than gmail. In true google fashion, as soon as people started to like it and use it, they unceremoniously killed it.

You say stagnant, I say stable.

Agreed.

I can't think of a single feature I personally think GMail is missing (and arguably some features I wish they hadn't added).

I do think their settings panels, and their filter management in particular, could sorely use some UX improvements.

Interested, would you want gmail to do anything additional? Aside from a modernised look every now and then, I can't see that there's many additional features you might give an email client?

If you have the resources of Google available to you, I'd think something could be done to make email as a whole less bad. I get so many emails that inbox zero is NOT at all realistic. You could argue that's not Google's responsibility, and you'd be right, but it means I use Gmail less, which is at odds with me using it more and Google getting more of my attention.