Hello! I was thinking that would be a low hanging fruit for improving usability of trim, split and project brushes. Currently they auto complete operation when you finish the stroke. It would be nice to have “manual” mode so if you want to precisely adjust cut shape. So you could just draw the shape and then you would have simple green circle to accept the cut when you place it where you want. In Zbrush this is done by pressing space when you do similar operation and you can move the shape around. Polygon tool already works like this but it would be nice to have similar functionality for ellipse and rectangle. Also if shape is too small it´s really hard to click filled area of shape to make the cut since control points get clicked instead of filled area.
Yes, that‘s missing. A toggle between on the fly and validate would be cool!
My fav painting app Infinite painter is on a good way in This.
Almost same control for Ellipse.
Watch that control points are hidden while moving for better view on placement.
If you drag out the rectangle with pencil and tap and hold a second finger on Screen, a Perfect Square or Circle is drawn. The other controls are quite obvious.
Even though polygon rotating with two fingers is feeling magical in Nomad, I am missing control. It‘s magic without real control so to say. A little rotate icon will give more precision, for poly, ellipse and rectangle and it‘s easier to use for idiots like me. Tap second finger on screen while rotating locks in 45 degree steps.
What I seriously love in Infinite Painter is the stamp tool. Once you have found the size you want, tap stamp, the trim is done, but same tool with same size is still active.
You can move it again for a second action. If a second finger touches the screen while moving, the move is constrained to initial axis of move.
Stamp again and while you move it snaps on exact same distance between first and second stamp. This way exact distances between stamps are done on the fly.
Well this is not all available in Infinite Painter, but maybe in Nomad soon? That would be a blast! 
yeah Infinite painter is also my go to for 2d stuff on ipad. It´s just got very nice customizability. Clean and intuitive UI.