The black and white balls create a permanent extension (thingy). Have balls that create a woodlike grain permanent extension and after a set time, Gooberries grow.
The green ball that is retractable and extended to join with others to form a structure (not the free moving green balls) closest to the berry would (after a given time) unattach itself from the structure to eat the berry and return to its original position, if at all possible (assuming the structure has not collapsed).
Berries can vary in colors doubling the weight of the gooball that ate it, tripling its weight, solidifying the entire structure, etc.
Players can spot the berries to pluck them to strategically place them, toss them aside or collect them for bonus points (gooberries have a short lifespan maybe).

OK, so is this an idea for a new Goo ball? Or, like a mod or something?

I've played the game from start to finish (loved it!) and really, really enjoyed the music and background info. This was just a suggestion for future development, if any... and yes, I guess it would be a new gooball.

Ah, cool. In that case, you might want to post it on the goofans.com forum, because 2D Boy is unlikely to build onto WoG, through a sequel, prequel or otherwise.