After working with FigJam on a wireframe mock-up, here is the list of features I would like to see:
Quick undo button on bar
Quick redo button on bar
Ability to hide bar (then hover over and it reappears)
Ruler tool to draw straight lines
Ability to add effects to objects - gradients, partial fills, opacity, etc.
More thickness settings for marker
Better stylus support (or stylus support mode at least - i.e., trouble with hand marking while using stylus)