When you think "pro desktop," you probably don't think of the iMac. You can get one with a pro-quality display and a high-end (consumer) processor that will do the job for many tasks, but you likely wouldn't buy one for massive 3D modeling or video projects. However, Apple might have you changing your mind before long. As part of a roundtable on the future of Mac desktops, the company's Phil Schiller has promised that there will be iMac configurations made "specifically with the pro customer in mind" later in 2017. And before you ask: no, this doesn't mean a touchscreen-centric design akin to Microsoft's Surface Studio.
Schiller claims that touch input "doesn't even register" on the concerns brought up by pros. They're thinking "performance and storage and expandability," he says. This probably doesn't mean tearing open your iMac like you would a typical mid-tower desktop, but there are obvious candidates for improvements. You could see faster processors (such as Xeons or Core i7s with more than four cores), better graphics chipsets than the mobile parts you see now, higher-capacity storage options and, of course, Thunderbolt 3 ports.
There are limits to what you can stuff into an all-in-one chassis, and it's reasonable to say that the iMac won't scratch the itch for many pros -- these are the people who frequently want 12-core chips, multiple graphics cards and as much memory as humanly possible. But between this and the planned 2018 Mac Pro redesign, it's evident that Apple is acknowledging complaints that it isn't taking desktops seriously.