Writing tools shortcuts
If there could be a tray where I can add 4-5 various colors of pens and highlighter mimicking a real physical object that I can just pickup and write. For example, I could add a red colored pen with fine thickness, a black colored pen with bold thickness, yellow highlighter pen with certain thickness and automatic straight line.
In that way I can jump between each tools and options without having to turn on/off auto-straight-line function or adjust the thickness every time I change a writing tool.
You can let this function to be optional that users can toggle on/off in settings if you are afraid that it will complicate the app too much.