Amazing what you can see when you look.
Irv Bromberg at the University of Toronto discusses an adjustment. Currently there are 13 months in 7 years out of every 19; the new formula would involve 130 leap years out of every 353.
As he clarifies, witnesses only determined when exactly the new month would start; the Sanhedrin could decide whether to make it a leap year.
As far as the problem with the existing system:
... the average moment of the northward equinox ... drift[s] progressively earlier in the Hebrew calendar year, at a rate that is currently about one day earlier per 220 years. Until today the average equinox has drifted about 6 days earlier than the start of Nisan, in Jerusalem.
He observes that if we continued the current system for another 4000 years, we would actually get Passover not in the springtime anymore, violating a key design requirement. (My comment -- fortunately Hillel said "eh this will get us by for 1600 years and then we'll see how it's going"; so we won't be forced to hit that point.)
See Wikipedia for more.
Just for consideration, the average length of a solar year, in days:
- As measured by scientists today: 365.2424
- As used by the Gregorian calendar by most of today's world: 365.2425
- As the Hebrew calendar has been doing for 1500+ years: 365.2468
- As used by the Julian calendar in Europe until 500 years ago: 365.25
So Hillel's approximation was good for its time, and had an expiration date that allowed enough time to develop an alternative, (?), but not long enough that it would fail its design requirements. Not too shabby.