The creation of the Big Daddies and Little Sisters required, as I recall, a fair bit of effort on the part of Ryan's scientists, and presumably the creation of a new breed of "Big" would take similar work. But Ryan and Fontaine are dead and Rapture, when last seen, was collapsing into anarchy. The only one left (that we know of) who might have the technical chops to make this happen is Tennenbaum but her commitment to setting the Little Sisters free is at odds with the idea of kidnapping them for reasons unknown.
So... what's next?
So... what's next?