I wouldn't want to add a slime girl transformation for a few reasons. One is that it would likely overlap with the mouse transformation in practical applications (you use the mouse to crawl into small spaces, and a slime would be able to do that as well), plus one could imagine being able to stretch over gaps and other stuff which aren't planned.
We are also near the end of the development, adding a new transformation would be too hard to fit now (all transformations are used in puzzles and exploration).