The iMac is already ridiculously powerful for its form factor. It's as fast as a mid-range gaming PC but more compact than an average 27" display. It will not be possible to have a desktop GPU there without seriously compromising the form factor. So, no, I don't think that we will ever see an 'iMac Pro'. At the same time, given the advances in computing, an 'iMac Pro' would be probably completely unnecessary few years down the line - as a midrange CPU/GPU should be more than sufficient for all kinds of work. The Mac Pro is good for content creators, which need to process ridiculous amount of data in real time. For most other things, its quite an overkill.