Either Like its already done: a seperate Feature, where you can choose lines or circles, but in Addition to the original Feature that should be able with the marker/Highlighter Funktion.
Or another way would be, Like goodnotes, when you Lavendel your pen in the Display for a few Seconds, then the Line/circle/recktangle...is automaticalli perfetionated with straight lines.

Hope this is possible, because inuse Android iPad and really Like your App. Sorry for misspelling, I'm Not a native english speaker.