bitflipper
Admittedly, I don't see any contenders on the horizon - yet. But there are a couple that could be Kontakt-killers if they played their cards right, starting with ditching their frickin' dongles.
I think Halion 5 could be a contender, at least technology-wise, if Steinberg would lose the dongle. I don't have first hand experience with Mach 5, but it's my understanding that is as good if not superior to Kontakt. Once again, the dongle keeps a large population of candidate buyers away. So, yeah, there's two coulda-beens but for dongles.
It's probably dangerous to even suggest it here, but ST 3 might be that condender. No dongle, and presumably a registration/activation procdure that's no more onerous than NI's.