Instead of having a physical touch button to mimic alt and shift, maybe draw inspiration from iPad only apps like Procreate to make the iPad Illustrator as useful as the desktop version.

Couple of ideas:
1. Decrease the affect palm movements have when using Apple pencil, and only respond to gestures and menu-taps when using Apple Pencil.

1. Make it an option to use the physical gesture button or not.
   2.a When not using a pencil it could be useful
   2.b When using pencil, using finger gestures instead of a button would give more and quicker options.

2. You can make it so holding down one or two fingers anywhere on screen does the same thing as either alt or shift like on Desktop

3. Use gestures for quick acces to menu or tool options.
   4.a For example slowly swiping down with 3 fingers could mean picking different tools like pencil tool or selection.
   4.b make toolbar customisable

Then some other small things

1. Fix the shape builder tool so it works more like the Desktop version.

2. More recolour options like when using shape builder on Desktop.
   6.a Add a drag and drop function for colour when in selection tool for quicker colouring

3. Make use of the top left bar for more features.

4. Make a gesture shortcut for colour picker tool like pressing down with pencil