I know this isn't a satisfying answer but
just about anything. Stalin tried to create an army of human/ape hybrids.
Why did Russian zoologist Ilia Ivanov try so hard to create babies that were half-human, half-ape?
www.newscientist.com
You don't think DARPA has their arms up to their elbows in this shyt? Imagine having access to near limitless samples of human DNA. Now imagine what you could do with that and CRISPR the gene splicing tech that dropped within the past 10 years. Throw some AI in there to speed the process up and before you know it, it's Gattaca time...