So far affinity photo for iPad is looking great! This is almost exactly what I have been looking for ever since the iPad pro first came out. I know it's mostly positioned as a photo editing app, and not a painting and illustration tool but there are a couple of things that would make it perfectly suited to painting if they were there:
Easier colour sampling - I've seen this mentioned in other threads for the mac version, but I can't really overstate how important the eyedropper tool is for digital painting. 90% of the time in procreate I am just sampling and painting. In Affinity I have to tap the eyedropper, then tap the brush again - the ability to toggle between brush and eyedropper is really what's needed. Whether that's a button on screen like in procreate or holding down the alt key like in Photoshop.
Customization the key-commands. I'm sure this is already on your list, but being able to do absolutely everything from the keyboard would free me up to always have one hand for switching tools and one hand on the stylus at all times
Pressure sensitivity for opacity - I was amazed when I saw all the controls for each brush, but opacity is the one thing that's missing in there. I know flow is similar, but I find sometimes opactity is what I really want to vary
Disable finger input for brushes all together: Palm rejection works okay, but in every app I've ever used it's still not good enough to 100% prevent random input from my palm. A lot of apps have added the ability to make it so only the Pencil can make brush strokes, which solves this problem
Rotate and zoom with multi-touch without 'rotation mode' - when I am painting I'm basically constantly rotating the canvas, and it would be more seamless for rotation to be always on like zoom for multi touch gestures
Undo with multi-touch: Other people have mentioned the two-finger tap for undo is so ingrained in my me by procreate that I find myself doing it in every single app (sometimes when I am drawing on paper too!) - I think that would be a good feature to copy
I hope it's not too annoying that every thread is filled with these little complaints. The truth is Affinity photo is a great app, it brings almost every advanced feature I have been looking for to iPad. Especially masking! I look forward to seeing how the app progresses now that it's arrived.