Pat,
I'm working off memory so my facts and stats may be a little off, but here's what I recall:
I believe the original Greist Estate bequest was 540 acres or something like that.
36 holes originally were planned; however, due to the enormous expense of the first 18 ($495,000 in the 1920s), the second was not built.
At some point, the university sold 250 acres or so of the estate; the remaining property does not contain enough room for another 18.
That said, periodically talk comes up of building a third nine, most recently I believe at the time Rees Jones came in. Golfers would welcome another nine as the course does something like 50,000 to 55,000 rounds annually.
Never say never, but I say it likely will never happen. The uni will not out up the money, so it will take a bequest. And the university even then may discourage such a gift, for they probably wouldn't want to deal with the addl OpEx.
And it would be very expensive the build and maintain: the remaining property is (surprise) severe and drainage will be a challenge; much if not all of the new nine would be built downhill of the existing course and if memory serves a bowl or amphitheater type topography.
I could be wrong on the details, but that's the gist of it...
Mark