A few ideas

[kais-grati]

• Hold pen to auto make shapes
• Dashed borders for shapes
• Highlighter pen
• Auto save on change
• Strikethrough feature where you scribble on text to delete it

[Doublonmousse]

• Hold pen to auto make shapes
  -> stroke straightening when holding pen still after stroke #384 and feat: long hold pen for lines/shape recognition #1175 for a draft PR
• Dashed borders for shapes
  -> feat: shape styling options and easier highlighting #1210 (in progress) and Different line styles #852 and [Feature Request] Shapes with dotted lines #375 for the issues
• Highlighter pen
  A pen with a transparent color isn't homogenously transparent, but leaves less transparent circles #891 and Add an highlighter that also works on background pdf/images #449
• Auto save on change
  This one doesn't have an opened issue.
  I think it wouldn't work well if saving uses the current file format (saving the full state on each pen up event then compressing it) as it will probably be too slow and compute heavy. Maybe changing temporarily the format to be uncompressed whilst doing that then re compressing it when quitting would make it easier to do. Though having a diff system instead of a full save would be faster.
• Strikethrough feature where you scribble on text to delete it
  Scratch to Erase #789 ? Maybe scratch is a little bit better than strikethrough ? This has to be optional and having the "math fractions erase everyting bug" wouldn't be great