Least likely to be licensed?
To my astonishment, Hourou Musuko (by the author of Aoi Hana/Sweet Blue Flowers) has been licensed in English. Not the absolute last series I follow that I ever expected to get official translation, but darn close to it. I mean, it's a GBLT story about the friendship between a pre-op MTF and pre-op FTM before and during puberty, done seriously, without fanservice and realistic adolescent angst. That ain't the sort of stuff that gets picked up. Unless, apparently, you talk indie art-comix house Fantagraphics into starting a manga line.
But this makes me curious -- what's on your list of titles least likely to be licensed?
Keep in mind that history repeatedly shows that sex, violence, and gore in themselves are not barriers to getting licensed.
My entry: Saint Young Men. It's hilariously blasphemous, but two-thirds of the jokes rely on knowledge of Buddhism that westerners don't have, and of remaining jokes based on Christianity, about half get something, er, not quite right. The only America publisher with the heuvos to take this on, Dark Horse (Viz certainly doesn't have 'em), is one that Kodansha never licenses anything to. Who knows, maybe Del Rey (Kodansha's go-to licensor of the moment) will surprise me and tackle it, but I'm not holding my breath there.
Runner-up: Tokyo Akazukin. Because, well, yeah. Not just sex and gore, but f-ed up sex and gore.