Hi,
Using Kdenlive 17.04

Except if I misunderstood how to use Kdenlive, I see a feature to be implemented.

One may want to render a project in different resolution outputs, all having same DAR(Display Aspect Ratio), for instance you might want to render your high quality 16:9 4K project into a low quality 16:9 720p.

Although, both have same aspect ratio, to my knowledge, this is not possible, take for instance the "position and zoom" effect, as the effects seems to be a fixed measurement (in pixels) and so the skrinking homothety (from your 4K base project to 720p) is not calculated, you need to rebuild a new project for 720p!

Of course, your project ought to always start from the highest resolution down the lowest.