May this already exist and I don’t have to use it, but here we go:
I would love to preview mobile versions in my phone (ipad would be nice too) emulating different devices with a pixel density 1:1. I mean. I don’t own several phones, so being able to emulate other devices may help evaluate how the font, buttons, etc size will work on them. I don’t care if the size of the preview is smaller or even overflows the phone (it should be able to be panned, for sure).
What I usually do is to have an artboard with the size of my actual phone and placing the elements just like in the bigger/smaller phone to preview. But it’s not a fluid workflow.