I think it stands to reason that Canon will release a new ultrawide zoom sometime soon, whether its a 16-35 update or a 14-24. They just updated both the 24-105 and 24-70 lenses, as well as the 70-200. That leaves the 100-400 to update sometime in the next year or two.
Of the primes, the 35L is the oldest by almost 10 years. The 135L is also in that timeframe. So, I'd say those are the next two L updates. That said, I think the 135L is still selling well, not sure it really needs an update yet.
The100-400 seem to be the most likely update, since Canon has updated all their tele lenses in the last few years. 35L and ultra-wide probably come after that. But, with a lot of good L lenses, you may see them update their non-L lenses before. I think the 50mm IS update is coming, and they've refreshed all the kit lenses with STM