Another gameplay would be to collect stuff, by creating a way, but you'll only have a limited amount of ink on every level.
(J'me rappel plus si tu parles français mais bon)
J'ai aussi pensé à un crayon qui coulerais, et qui pourrais affecter le niveau (cependant j'pense pas que ce serais fesable :S)
Oh wow just playing with the yellow pen gave me anoter idea:
For the "storymode", the levels could also contain spaces where the player will have to stop the picture, and draw a rope to swing the "hero" on the other side (eventually he'll think about drawing a way, but maybe something could block the other "inks" )
you should implement both compression and stretch-type springs, as well as water/fluid. a duplicate tool would also be great, as well as a line/shape tool to allow for slightly more precise drawing.
stretch type springs will be maybe implemented, but I don't know currently. Duplicate tool and line/shape will be implemented but with a special editor