As for ayu, it would be up to avex whether they would ever cater to the U.S. market.
And Twisted is right, Hikki won't be allowed to take any creative risks and will probably just stick her to singing standard R&B material. It's not like a full blown music genre crossover, like it was for "Latin" music with Ricky Martin, Antony, Shakira, etc where they were able to use and popularize their own creativity.