Obviously it cannot be trained, but does it ever improve over time? Surely vision is something that improves with age and experience?

I see it as Judgement improves a player's ability to understand the game, and a given situation better, and improves over time. The ability to see things other players can't is god given and doesn't improve over time. But sure, it could be argued either way.

Fair point. Could this be added like the increasing speed/aggression in the near future?

I used to think that speed couldn't really be improved til I read about Frank Lampard Snr and Jnr in Harry Redknapp's book, practicing sprinting a lot after training, and the difference it made. Being able to increase a players speed by 1 is justified by that, and I think aggression is similar in this respect.

But however hard a player trains he won't be able to see the things that Eric Cantona or Kenny Dalglish sees, so I like Vision being the only stat that can't be improved at all.

Another issue is, stats like Passing, Stamina, Vision are made up of other hidden stats like 'Talent', 'Athleticism', 'Confidence', 'Developed Passing' etc. Because of the way Vision is made up it would be difficult to improve it without risking increasing other stats.