Veigar "DFG Compensation"
Personally I think Veigar is perfectly fine right now, but if he were to get a buff I would add it to his only difficult to use/master ability and the effect being easily avoidable as well as long the Veigar is not super good (like Veigar super main in Masters/Challenger) with his accuracy. The proposition is something like: Event Horizon - stunned enemies gain a debuff for 1.5/1.75/2/2.25/2.5 seconds (same duration as his stun, reason to why it is a debuff is that it doesn't lower with tenacity, but still removed with Quicksilver Sash) reducing the enemies' magic resistance by 12/14/16/18/20%. This will basically grant Veigar more damage (and utility) without directly giving him more damage (which he doesn't need). Flat shred is not recommended both because Veigar is suppose to be a late game champion and just makes squishies die faster, there needs to be some kind of counter play in allowing the chance for the squishy to survive to fight back. To be blunt it is a death trap for a reason.
and lower cooldowns or longer range
. Neither of them had Veigars short range or mana problems and much lower cooldowns.
before they hit you (hey Zhonyas works against Zed ult so he can be strong... hey it also works against Veigar ult so he should get to be strong too... no but seriously.... Veigar is mostly fine)