You know, regardless of whether it's considered cannon by us or Disney, the fact is that Bioware made the original Kotor games that had HK47. Bioware was bought out by EA long ago. EA bought the license to produce Star Wars things from Disney.
Technically, in my opinion I guess, EA can pull from anything star wars related that they own. Including Kotor. I guess Disney can still say no, but I don't think they care. At all. See above for why.
About Starkiller, yeah. Thatd be great. He was an awesome character an I'd like to see him.