I think it would be very interesting and add depth to the champion, because it would be a LOT about mind games, which also fits his theme, like he's an evil yordle toying with you.
Make the cast animation on Veigar itself more clear (so you know he's launching E), but don't show where he would land the cage. Just guess it, while Veigar has to guess where you'll go.
I believe it would add a lot of play potential, having something powerful but tied to predictions/mindgames.
I would not do that as a straight buff however, as Veigar isn't performing that bad. I would increase his E cd by 2 seconds in compensation, which is also I think a healthy way of giving him a higher window of weakness.