It's certainly been coming for a while.
In files that shipped with a new beta build of Valve's Steam marketplace, the SteamDB third-party tracker picked up a couple of concept images outlining some visual changes to the digital storefront.
— Steam Database (@SteamDB) February 1, 2017
It's about time: while the store page has gotten a visual refresh in recent years, the Steam client itself hasn't changed a great deal. The inclusion of Big Picture mode made a big difference if you were playing in the living room, but it's not the default for most people. That's what Steam seems to be doing though: taking some of the stylings of Big Picture and merging that into the regular client.
There's also a peek into what might be happening with individual game pages:
Obviously Metal Gear Solid isn't getting mashed up with Dota 2, but you can see the general idea. It's a lot less cluttered, there's more of a focus on images on the sides rather than text boxes.
The files were originally available in the /clientui/ folder of the Steam install - for those on the beta branch of the program - but Valve quickly pushed out a new build that removed the images. And it's worth remembering that this is really just concept art for now, not a final indication of what the Steam UI or game pages will look like.
But the implications are obvious: Valve is giving the Steam client a makeover, eventually. We don't know 100% what that will look like, but the images from the earlier beta build today at least give users an indication of what to expect.