Given that PowerApps is formula based, I'd find it really helpful to have an equivalent of the VBA Immediate Pane - i.e. somewhere I can type/copy in a formula and see the result right away.
Further, a tool-tip (or similar) within the formula bar that gives the value(s) of the highlighted expression would greatly improve the design experience.
Similarly, a variable watch list would be nice. There is a variables tab in the Designer, but it would be better to be able to see multiple variable (and ad hoc expressions) values while still being able to view the application itself.
At the moment, I have to add labels and enter expressions or variables there, then remove (or hide) them again after 'debugging'.