Frame Buffer and region render option for productivity in pathtracing!

Dear Epic!
I am using your exceptional render engine but coming from an offline renderer, sometimes is painful not to have specific tools that objectively make our life easier and faster.
One of this is a frame buffer like in vray/ corona etc. And ofcourse that only makes sense for the pathtracing mode. It is so painful every time to have to render the full scene in case you want to make a small change, or not to have an option to change the resolution and save directly a fast render from there. I am begging you please intigrate this to your future updates or gives us the option to select a specific resolution for the viewport. My workaround for now is to use my second screen on a lower resolution (1920x1080) so at least every time i render the same resolution directly from the viewport and i dont have to use the movie render option.
These are essential tools for productivity and i understand that the engine works different and is very difficult to initgrate a light mixer option but can live without it for sure.
I am so looking forward to it and i am very sure that i am not the only here that will benefit from that…