As this is not a 50mm IS refresh rumor (which will undoubtedly not be an L lens), if there was a new L lens on offer, I'd guess the one most in need of a refresh would be:
35mm f/1.4L - 17 years old
135mm f/2L - 19 years old
Those aren't the oldest L primes, but they are a strong combination of old and in high demand. My money would be on one of those.
Is the 135mm in need of an update? I haven't heard anything put praise for that lens.
Indeed I rented it once and it's a really nice lens. Just a little too specialized for me. Maybe for once they want to try and announce it before Sigma does? Given the quality of the Canon 70-200 IS ii, the main purposes for that lens is super low light or sports. 35 or 50 is more standard and I would think more in need (especially the 50)