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.
Crudely, there are three camps with the 135L:
- The lens is stellar and does not need modifying.
- The glass is fine but IS would be a great add.
- Sony has a 135 f/1.8 and we don't. Also: Waaaaaah. (My rebuttal? Our autofocus works.)
And deep down, as legendary as lenses like the 135L and 200 F/2L IS have been, old is old: there's always room to improve resolution for higher resolving sensors, reduce flare, reduce distortion, reduce chromatic aberration, reduce weight, improve weathersealing, offer or improve the IS, etc.
The bigger question is whether those improvements are worth putting off my future non-L 50mm f/nooneknows IS USM. And the answer is no. Gimme my 50, dammit.