If thunderbolt 2 on the iMac was capable of being bi-directional, then I assume thunderbolt 3 is capable of doing the same. I understand that it makes more sense to have such a feature on the iMac via the Target Display Mode that would allow a user to take advantage of the iMac's gorgeous display from an external TB/DP output source (such as a MacBook Pro).
However, I've been in many situations personally where I really wished that I could use my other laptop or tablet, current or older, as a second monitor. One of the "decent" solutions available to allow for such an option is duet; a software solution that allows the iPad to act as a secondary monitor, wirelessly or via USB, to the Mac. Similarly, many has been trying to achieve similar results using The Remote Display feature. In my experience, duet works best, but even so, it's very limited and not satisfactory to my needs.
In other situations, like gaming, people have been trying to take advantage of "remote play" to do the same. However, the lag and the quality of the stream, as impressive as it is today, just not enough to consider for a true pleasurable experience.
I believe there's a need for that. A video-in port that allows the user to use the display as an external monitor. I believe that it will significantly make the device appeal more to a far wider segment than just those who are interested in computing (laptop or tablet) given the form factor, built quality, I/O, and the long lasting battery. Actually, it'll be the only one in the market, to serve both as a tablet, laptop, and a mobile external monitor.
I think even photographers and filmmakers (content creators included) might be interested in such a feature to have on the go to hook their cameras in and benefit from the larger higher-resolution color-accurate viewfinder.
The gorgeous display you claim to be the best in the market will only increase in value of the device for what it actually costs, and even more with such functionality. It'll prolong the device's life significantly.
I'm not an expert when it comes to technical details and difficulties to make it possible. But if it's possible, I think it's a must.
Some people would justify the price of the 5k iMac (when it first came out) just because of the display. It's really unfortunate that Target Display Mode is not available anymore.
P.S. I'm not sure when it would be appropriate to tag the Creators, Senior_Creators, and Team; and although I'd really love to hear their opinion regarding the suggestion, I decided to just post it without any mentions to avoid unnecessary spam. I'd appreciate it if someone official would tell me in what situation it'd be appropriate to do so.