Well, it is a "bug", sort of - but now a software bug, but a blue bug.
To unlock level 25, you have to solve the secret of the blue bug.
Here are some hints (hidden because of slight spoilers, select with mouse to make visible):
Have you ever wondered why there is one single blue bug in each doodle, in addition to the red ones?
Have you ever wondered why there is also a single blue bug on the level select screen?
While playing the doodles, keep your eyes open for something that might be significant in some way (more obvious in some of the later doodles).
Have you noticed how on the level select screen the nodes for the levels that were still inaccessible are shown in black&white and only the unlocked levels are in color? Once you have solved the last level (doodle 24), have a good look at the level select screen again...
Regarding the DLC question: We are currently investigating the possibilities, but I can't make any announcements or promises at this time.