Graphically it would generally be taken as a single line. So, what ever gameplay you could come up with would need to be contained on a single line of display. If the line changes position in a direction perpendicular to itself then you have entered 2D space. Color changing and contraction and expansion of the line are all I can think of for display elements. Sound could play a big role in feedback to the user, as could haptics. I can think of a few implementations that would techincally be a game, but I don't see much in the way of longevity. Sound and haptics are, however, additional dimensions.