I had really hoped that J would be the bassist... (still hope so) but I'm sure Yoshiki has considered the possibilty, maybe even invited J (since Sugizo was invited). I'm suspecting J probably prefers to do his solo work, which I love, by the way.
There really doesn't seem to be an ideal bassist that could join S.K.I.N. Considering that Taiji has been sick (read somewhere that he's in a wheel chair now?!?); heath is not really on the same page (level of famous-ness); and I doubt they will recruit any bassist who's currently involved in a band. (Even Miyavi was a long shot... the age and experience difference makes it quite awkward for me to perceive the four together... :/
This reminds me of many, many years ago, when I was a passionate teenager (^^;

and there were surveys (or more like daydreams) on what the ideal band would be. Mine was
Vo. Gackt
Gt. hide
Gt. Sugizo
Ba. J
Dr. Yoshiki
I wasn't too far off! LOL.