I love the way you 2d guys have handled the story using signs and the signpainter, but like all craving lunatics I want more.

I would love for the levels to have hidden signposts, at weird places like the opposite side of the pipe, and tell a different story, maybe of individual balls or just fun facts.

I think this also can add a little to the replayability, because I could explore a little as well.

This thing also gives me an idea about secret levels that are unannounced, no arrow pointing towards it.

Just throwing out ideas here

Hidden signposts/signposts that are in a place you wouldn't usually go would be cool. Maybe if they contained utterly useless bits of info/conversation, like the 'How did i get up here?' sign. More signs like that in obscure places would make you want to explore as well as reach the end of the levels.

Good idea :)

Great suggestion Torquai!  I know I'm always interested to see what the next billboard in WoGCo is going to say.

Maybe secret pipes in certain levels that unlock bonus levels?
You could do stuff to hide them, too.  Maybe make a big iron gate around one and a string hanging down next to it.  You would attach a structure to the gate, which would weight it down, causing the door to lift.  To  make it even cooler, make it so you only get the number of goo balls you got through the pipe (If you only get 1 goo ball through, in the bonus level you only have 1 ball).