From the Kronos Quartet's David Harrington's blog post on Q2:

"We are very excited about his next piece for Kronos. His String Quartet No. 6"

In the symphonic world, after Beethoven, the big reckoning is writing a Ninth Symphony.  Similarly, I think for any 20th Century composer of string quartets, the gold standard and "large shadow" would be Bartok who wrote six of them.  As such, Glass hasn't written a numbered quartet since 1991 with No.5.  

This is interesting because Glass has written string quartet and quintet music including his score the film BENT (performed on the soundtrack by the Emerson Quartet); for Dracula for Kronos, and the opera IN THE PENAL COLONY for string quintet and two singers.

But if the string quartet sections for his film score to MISHIMA became a numbered string quartet (No.3), why didn't BENT find the same destiny?  It could be a judgement of musical quality: I find Mishima to be of great musical quality and inspiration where I don't get that same kind of cohesiveness from what became titled "Suite from Bent for String Quartet."  In any case, I believe Bartok was on Glass' mind when he the decision not to give a number to the piece.

While we're talking about string quartets, it's also of interest to note that Kronos Quartet has premiered all of Glass' numbered quartets, including No.1, composed in 1966 but premiered twenty years later in 1986; No.3 "Mishima" for the film recording in 1985; No.4 "Buczak" in 1989; and No.5 in 1992.  I'm not actually clear on when the 8 minute miniature "Company" for string orchestra or string quartet, became No.2.

In the end, I'm sure this piece is probably a ways away.  THE LOST, Glass' new opera for the new Linz Opera house was completed a few months ago.  That was originally scheduled to premiere in September for the opening of the new house but was delayed until April.  So Glass' opera about Walt Disney called THE PERFECT AMERICAN will premiere first in Madrid in January.  The composer is in the midst of composition.