The ability to have the Formula bar moved to the side or top according to preference. Sometimes it would be useful to see the whole PowerApp while editing the Formula. Just to check that you have referenced the right control!
The most frustrating thing about PowerApps is the limitations of pulling toolbars into place. The right-side properties pane is expandable, the left-side navigation pane is not, and the formula bar expands to cover everything you're working on. I find myself expanding and collapsing the formula bar constantly. Why can't it work like Excel so that when you drag it down the side panes and the app viewing area all make way for it? I'd leave the formula bar open with 5-10 lines showing all the time.