I haven't played since beta, but for around 1-2 years I played veigar only, and I stopped playing the game when dfg removal happened in wait of a compensation. It may not be for the longest of times but 1-2 years is still a good amount.
Let's clarify my thoughts on him being an anti-mage. Firstly let me say, I don't understand why people consider him to be one. He's an anti-anyone who builds flat Ap with no utility champion if anything. So people like leblanc was what veigars ult was truely good against.
Now here's why I don't think he should be an antimage or even considered one. There are a lot of champions who are played mid who are AD or don't really build that much AP. It's actually to the point you don't even consider the bonus damage from enemy AP, it's just too inconsistent, and if something is too inconsistent you just don't add it to the 'plan'.
Referring back to pre dfg removal, the general rule was, if you didn't have a decent amount of health and/or MR, veigar would instantly kill you. It didn't matter if you were Zed, khaz, leblanc, ezreal, caitlyn, oriana, syndra, or wh ever, without health and/or mr you were going to die or you were going to live.
From all my experiences I've never once noticed the bonus enemy ap damage really mattering. I've played veigar at a fairly high level for a couple of years and I've never considered tabulating the enemy AP as damage.
I never looked at the enemy AP and tried to figure out if it'd kill them. I never considered him an anti mage, I considered him an anti-anyone with low hp/mr, because that's what he was. He was a nuker, no reason to attach anti-mage because he was just as good against the AD casters.
Now what are my proposed changes? I've thought about it a couple of times on how to make him more fun with his current skill set, but I haven't really thought about a real rework.
His passive needs to be changes. As it is it sucks, and I've never liked it. It gives you much less than you think when you consider his overall mana costs. Useless passive.
His E needs a rework just flat out. Just keeping it as a cage I would add augmentations to his pre existing skills.
When his Q passes through the walls they penetrate all targets within, and maybe gets a jayce treatment in that it speeds up, and gets bigger with more damage till it hits the other side of the cage and disappears. This would fix the issue where people can just kill you in return if you trap them inside for them to 'dance' around.
When you use his W inside the box, drop time is instant, maybe more damage.
R change, I'd like to have the enemy AP scaling removed, it was always a stupid mechanic, and then increase it's scaling up to 130%, then have it do 150% when hitting an enemy in the box.