I'm a surgical resident, and I've used almost every form of note taking, app and study method available in my 30+ years of being a professional student.

I've used Notability for a long time, but switched to Goodnotes because of the feature set, community, and one-time fee model. There are a few key missing features that make me switch back and forth between the apps.

Disclaimer: I realize the amount of effort it takes to execute new features and believe people should be compensated for their work. I would not be opposed to in-app purchases for the additional features proposed below.

1. Gestures A. 2-finger tap to undo. Gesture recognition is poor. I find the app can't tell the difference between undo and zoom. Ideally, tapping once rather than twice to undo. I find it's faster to tap to undo on my iPad than it is to tap the button on my Apple Pencil to erase. B. 3-finger tap to redo. This gesture is missing, or input recognition is not captured. Again, the option to tap with 3 fingers once vs twice is preferred for expediency.
2. Inefficiencies A. Missing color dropper. There are over 450 separate requests for this and it hasn't happened. "But we have hex codes" you may think. NO. Try to white something out on anything that isn't 'FFFFFF' shade of white. Hex codes are inefficient, especially for students taking notes during a lecture. To this effect, increase the max pen diameter. It's really frustrating and time consuming to have to scribble something out vs redact it with one stroke. B. Improve shape recognition, and add the option to fill with a solid color. more than 50% of the time I draw a circle and get a rectangle, and vise versa. Imagine how frustrating this is when I'm trying to draw something complex like pathways (complex flow chart) or a mind map. Also, including arrows as a recognized shape would be game changing. i. Once a shape is drawn, give the user an option to change the fill (solid, transparent (or even better, a sliding transparency scale) line color and line thickness. C. Give users a customizable tool bar where they can add their customized writing instruments (i.e. writing implement, color, diameter) and lasso tools. This would be akin to the analog of picking up a different color pen. The current system feels like the user has to open a box of colored pencils, pick the color, sharpen or dull the implement, test it out, and repeat until it's what they're looking for. It's too many steps. It should be as fast or faster than changing analog writing instruments. D. Instant alpha. Oh the time I would save! Having the ability to remove a background from an image would be phenomenal! i. Ability to erase portions of an image. I can draw on it, why can't I erase a portion of it? E. Elements. Seriously, I love this idea! However, there's a lot of room for improvement. i. Users should be able to choose to save a page of images/shapes/icons (think sticker sheet) as a .collection file. Having to cut out individual images is painfully tedious. ii. At the bottom of the elements drop down, organization of elements can be improved by giving the user an option to display all categories rather than horizontally scroll for days looking for the right category. Or, allow titles instead of icons. The some of the element icons are so small they're unrecognizable. iii. Element and shape scaling options are limited to the original ratio. Please change this scaling option to include independent adjustment vertical and horizontal dimensions (exactly like the adjustment options for images). iv. Why is creating color changing elements so limited and difficult? In order to have the option to change the color, the object has to be drawn, handwritten, or text directly input onto a page. Why can't I select text from my digital textbook and change the color? Or paste a black and white image (think simple icon line drawing) and change the color? F. Layering objects. Assigning objects to front, back, move forward or backward. G. The lasso tool options are fantastic. Would love an addition to select as a rectangle vs free form. A rectangle is faster when batch selecting. H. The option to change pen diameter on handwritten text or shapes. I. The option of changing the color of a selected object by dragging the color to the selected object.
3. Optional/additional views A. An optional view for rulers (think Microsoft word). B. A snap to grid to help us align objects and shapes. C. A book view where two pages are displayed side by side (like they are in a physical book). D. Horizontal scrolling that displays more than one page at a time. Like in vertical scrolling where the top or bottom of the next/previous page is visible, but in this case the left and right of the previous/next pages would be in view. E. Closing the gap between pages in vertical scrolling so it appears like a continuous document. This could be a preferences option or done by double tapping on the bottom edge (vertical scrolling) or right edge (horizontal scrolling) of a page. F. Zooming out further than the vertical limitations of a page. I will admit, this limitation drives me nuts. Zooming out further really helps with document navigation and it's disorienting not to be able to do this. Having to open the thumbnails menu to reorient is inefficient.
4. Navigation A. The ability to create links in text from an outline, table of contents, index, etc. B. Use the lasso tool or a gesture to follow a link in a document without exiting document editing mode. C. Files menu i. Option to display favorites first ii. Column view (like iOS) that shows the file nesting (i.e. parent folder --> child folder --> documents --> document preview) iii. Make vertical and horizontal document and file folder icons the same vertical size. Overall, the icons are too large. It wastes visual space to have one vertical document space out a row of folders/horizontal documents. iv. Color changing option for folders (i.e. Google Docs)

*Color changing options are a trend in the Goodnotes community, and for good reason. Using different colors increases pattern recognition, memory and recall. Using colors to organize can make a significant impact for users with various learning abilities and styles.