It would be great to see two new more sliders in the Transform panel, they would be called tilt vertically and tilt horizontally (or whatever name you may suggest).
These two will allow you to slide the left and right sides of your shot up and down or vice-versa. Same thing you could do in Photoshop with the Manual Transform tool if you hold Ctrl/Cmd + Alt + Shift on a side anchor. Think like breaking a rectangle right angles to make a parallelogram by moving two opposites sides symetrically and horizontally (or vertically).
The reason why it will be usfull is when using the Upright tool with 2 axes, the Rotate slider does not update the Upright transformation. The way to go around it is to use a third axe and not the Rotate slider. Sadly, the results can be wacky if you dont have any horizon line to reffer to.
These new sliders will unlock new possibilities, and solve many perspective issues. The slider values could be defined by angles but it would be more plactical to be a % of the image heigh and width.
Thanks for upvoting !