WWE officials currently have no plans to promote Samoa Joe and Asuka to the main roster.
According to Dave Meltzer of the Wrestling Observer Newsletter, both wrestlers are in NXT to add experience and some name value to the third touring brand. The idea is for them to help teach wrestlers being groomed for the main roster, which is why Joe worked with Baron Corbin and why Asuka had been working with Dana Brooke. WWE plans to eventually promote Brooke to the main roster.
Rob Naylor, who offered creative assistance at NXT and remains in touch with people in the organization, revealed on a recent audio show that WWE doesn’t have any big plans for Joe.
One issue for WWE is that if they promoted everyone who is good to the main roster, NXT would quickly become a shell. They need to have established stars who will stay as cornerstones for the yellow brand.