5. EF 100-400 f/4.5-5.6L IS USM
4. EF-S 17-85 f/4-5.6 IS USM
3. EF 50 f/1.4 USM
In more unsettling news, the new primes announced in 2012 certainly raised the price ceiling for consumer-level primes. Considering the price difference between the f/1.4 and f/1.2 version, an update of the 1.4 can easily fit into the lineup, whether it be stabilized (like its new peers) or not.
2. EF 28-135 f/3.5-5.6 IS USM
Currently Canon's only non-L EF standard zoom, it's been on the block for 15 years now. For the price, the optical performance and feature set and pretty much spot-on, but it could use some aesthetic modernization, a weight reduction, and an updated 4-stop (currently 2-stop) IS system . Having a shiny new version of this would make a tempting buy for new 6D owners who can't afford the 24-105 or 24-70 kit options.
Of course Canon could also give the 28-135 a much-needed brother or sister.
1. EF 50mm f/2.5 Compact Macro
The 50 CM is known for being sharp and distortion-free, but these days 1:2 magnification probably isn't going to cut it, especially when even the 18-55 kit lens goes to 1:3. Add that to the fact that the 'life-size converter' for 50 CM costs nearly as much as the lens itself, by which point the package costs more than 100 2.8 macro. Bundle that with the AFD focus motor and a huge amount of extension while focusing, and the lens hasn't aged well from a design point of view. A new 50mm macro lens from Canon that did 1:1 wouldn't even cannibalize any of their sales, since they don't have an EF macro lens shorter than 100mm, and would be for a different market than the EF-S 60mm 2.8.
Some lenses are commonly mentioned as in need of an update, but didn't make the above countdown for a few reasons. Main reason being that this is Top 5 Friday, not Top 10 Tuesday - I on;y have so much room in here. Nevertheless, here's why some popular choices were excluded from the top 5:
EF 400 f/5.6L USM
EF 50 f/1.8 II