Full screen mode
I have 27" Thunderbolt display and Checkvist takes use only tiny amount of screen real-estate. Could there be a full width mode for people like me? I also take quite lengthy notes so extra width would be a help.
Two pane is wonderful, many similar apps have two pane, for example: InfoQube, UltraRecall, ConnectedText, Quire and many.
Another idea is to use some of the real-estate, on the right side, to show the Notes associated with the current item. Instead of showing them under the item as though they were sub-items. In the same way that Google Docs/Sheets and MS Office show comments on the right side.
Lengthy texts are less readable. May be, we could consider using multi-column layout instead.
Personally, on my 27" iMac I always open a browser in half-width of the screen - so I have place for my IDE on another half.