http://i.ytimg.com/vi/VcnPJRohk3c/hqdefault.jpg

tl;dr: If Dark Matter applied a small knockback on impact, blasting the hit target away from the hit zone, it would allow Veigar to potentially stun people inside his rage-cage by knocking them into the wall. That would allow his stun to truly stun targets without them (e.g.) going afk or running dumbly into the wall, and reward skillful use of a still-hard-to-land ability.

The new Event Horizon

The big problem with Veigar's new Event Horizon ability is that, while it's still colloquially referred to as his "stun", it can't actually stun anybody who doesn't run smack into it. Right now, it's in that terrible place where it's quite effective versus people who have no idea who Veigar is, but almost never effective above that level. Whenever I land the new "stun", I don't feel satisfied—I feel like my opponent's mouse must have run out of batteries.

The old Event Horizon

The big problem with the old Event Horizon was that, while it did have a mechanism for actually stunning someone, it was a really awful mechanism. It basically turned this ostensible rage-cage into a very strangely shaped, well, "skillshot", to use the most applicable term: it was a game of lining the curve up with the enemy, and clicking at the right time without nudging the mouse to the side in the process. That's not a very satisfying mechanic for anyone.

A possible resolution: a reverse approach

It used to be that landing Event Horizon would reward you by letting you land Dark Matter on the stunned enemy.

What about a reverse approach? If Dark Matter were to knock the hit target back away from the impact zone (which certainly would feel themely), then landing a meteor on top of someone who's inside the cage might stun them. In other words, whereas before landing the stun rewarded you with a Dark Matter hit, this change would reward Veigars who landed Dark Matter with a stun.

Other benefits

To start with, I think this would feel quite themely: it's a meteor hurtling down from the heavens to strike with a very satisfying BOOM. It makes sense that would trigger a knockback.

It would also tie into other parts of his kit in an interesting way. For example, one classic Veigar move is throwing Dark Matter as he flees, trying to convince the enemy to veer off (and lose ground) to avoid the possibly literal tons of damage. (Well, at least a ton. Or maybe, to use a somewhat uncommon unit, a short* ton.) A knockback would reinforce that play pattern. Veigar's a low-mobility Champion, but this would give him some kind of defense (dependent upon a skillshot).

*: Possible short joke.