When designing a report it would be great to disconnect the data, temporarily, from the visuals to stop PBI crunching the data as you're moving things around. I feel this would speed up the experience. Then when you have your report design set up you can connect the data back up. At the moment ...
Gridline Color Customization
Just a small thing, and maybe there's a setting in the Power BI Desktop app that I'm not aware of, but turning on gridlines to assist with alignment is only useful in reports that aren't dark or have dark backgrounds. Would be nice if these could be customized.
Reports published to the service should fill the entire screen, like they do when published to the Web. The Navigation bar at the bottom when if fullscreen mode detracts from the content of the report. If you have in-built navigation you don't need it and should be able to hide it.