@meddler When considering changes to Veigar please don't skip over his most important skill
TL;DR: 
Veigar's E should work more like 
Lux's E, where it has a time delay which extends with range, but does not show EXACTLY where it will land--only the direction of travel (missile speed can be lowered). The time delay is justified, as opposed to an instant cast like before, however his dueling is currently non-existent and it is absolutely ridiculous that he currently cannot even use this ability to pick off competent enemies from fog of war, due to the indicator particles.
Currently the biggest problem with Veigar's stun is that it shows EXACTLY where it is going to be triggered. The 5.4 changes to add a delay were justified, even if it was a pretty big nerf to Veigar. However, when Veigar's kit centers around being able to land a stun to deal damage, showing exactly where it will land makes it simply too easy for enemies to avoid it, and any subsequent damage. This in turn, pushes Veigar into an extremely binary state where he sits back to farm ap with his Q, while occasionally roaming. He has absolutely 0 incentive to interact with his lane opponent, and simply needs to play safe for 20-30 minutes to hope his team isn't too far behind to achieve victory. Now, you may point out that there are other champions who have abilities with long cast times and/or that show exactly where they will trigger, however they differ significantly from Veigar.
Nami: Like Veigar, her Q (Aqua Prison) shows exactly where it is going to land and has a fairly substantial delay. Key differences to note: Nami was designed a support and has additional utility (E slow, move speed to get in range, knock-up after lvl 6) as well as anything a laning ally adds to better ensure her Q's land. If played as an AP Carry, Nami would have many the same issues Veigar currently has; however as a support Nami offers far more utility than Veigar can bring, as he only has one ability to add utility, and no way to set up that ability.
Bard and Ziggs both have ultimate spells which, like Veigar's E, show exactly where they are going to land. Key differences to note: Unlike Veigar, their spells trigger more quickly the closer they are cast to the champion. They can cast their spells on top of themselves to trigger the effect faster, if necessary. The delay which comes with distance, while making it easier for enemies to walk away from the spell area, also makes it more difficult to judge exactly when the spell will hit if they do not have vision of the champion.
Zac's E can have a significant channel time and shows exactly where he will land. Key differences to note: ZAC's E has a much more vague indicator than Veigar's E. Additionally, while it does show exactly where he will land, the spell is often used simply to get ZAC into the fight, with landing it being an added bonus. It does the job of getting the big, tanky blob into the back line to disrupt their positioning or cut off retreats, and it doesn't need to land to provide this utility. Furthermore, like Ziggs and Bard, ZAC's spell increases in delay with distance, meaning if necessary he can INSTANTLY cast it, at the cost of range, something Veigar cannot do.
Ekko is kind of ridiculous. His W, like Veigar's E, has a significant delay and can stun for 2.25 seconds. Key differences to note: Unlike Veigar, Ekko's W gives a cast animation, but doesn't show exactly where it will be until the parallel convergence field triggers, at which enemies will be subjected to a 40% slow. If they remain in this field when it collapses, they are also stunned for 2.25 seconds. For a stun that long, Veigar needs to have his E at rank 5, Ekko can do this from lvl 1, but as it requires a bit more setup, else it will be a 40% slow (which is still strong, as it makes it easier to keep them in the area to land the stun). Disregarding the passive effect of his W (Veigar makes up for that with his passive on Q), Ekko not only provides an extremely good zoning tool in his slow/stun field, he even gets a shield with insanely good AP scaling out of it. He is required to plan to be able to make plays, but nonetheless he can still make plays because enemies don't have the warning of EXACTLY where it will be. This means that, unlike Veigar, Ekko can use his W to catch people out of position with a Fog of War advantage, as if they don't have vision of Ekko they won't have knowledge of W's cast until it is already active. If Veigar hides in a bush and casts his E, enemies will know instantly from the warning particle.
Thresh's Q is comparable to Veigar's E in that it has a very obvious cast time. Key differences to note: Unlike Veigar's E, enemies don't know where Thresh's Q will be headed until the particle animation begins. ADDITIONALLY, this is made even more difficult by the fact that this unique skill doesn't turn Thresh's model to face the cast direction, allowing him to seriously mind-game his opponents. You'll see thresh facing you, winding up to hook you, flash away, and get hooked anyways because he aimed where he suspected you would flash.
Conclusion: In its current state, Veigar's E cannot be effectively used offensively. Much of his kit was designed around being able to land his stun, something he can no longer do. While other champions have abilities which operate similarly to Veigar's E in mechanics, they are designed with different kits and bring additional uses/utility which Veigar lacks. While he can farm AP more easily now, it is nearly impossible to harass a lane opponent effectively and the best strategy is simply to sit back and farm to get big for late game. Veigar, as a carry champion, needs to be able to make plays, and changing his stun so that enemies can react to it, but do not have the 100% certainty that they are avoiding it, is necessary for Veigar to become the niche pick he once was, without becoming oppressive.