Why Veigar's Ult Needs To Be Replaced
Reason #1: It restricts Veigar
Riot can't currently buff him too much because of the extra ap ratio from his opponents ap. Having a Veigar be able to burst ad champions as well as simply press R on ap champions would make him overpowered. This restricts his ability to do damage in general at the price of having a cheesy ability to kill the enemy ap carry with, providing you get the opportunity. In my opinion, I would rather see Veigar be able to play against all types of champions instead of only ap ones. Yes, he can burst all squishy champs, but not until late game.
Reason #2: It doesn't work with his current kit
This one is more debatable but Veigar's kit is based on stunning and killing the target. Right now he can't stun and his ult doesn't deal enough damage to compensate for not being able to land his W. Therefor this makes ult not nearly as useful as it should be.
Reason #3: It's not exactly "healthy"
Having an ability that scales off of another champions power is just really cheesy in my opinion. While its kinda fair for Veigar given how weak he is right now, it's also not fair to punish another champion for building ap.
Reason #4: Veigar loses to a lot of ap champions now
You can't kill'em if you can't stun'em... Until 45 minutes in where you can just press R, but is that worth it?
A completely new ult for Veigar could give him the boost he currently needs and new healthy identity that would make him fun to play and fair to play against.
My idea: (Wont happen but seems like a cool thing to think about)
Give him leblanc's ult. Of course this wouldn't actually happen because Riot wouldn't give two champions the same ultimate but its an idea. Two E's gives Veigar a better opportunity to stun a target and better defensive abilities as well. Two W's would make up for the burst lost from his ult. Two Q's would allow him to win trades in lane. All these things are still counterable and it provides more options for Veigar.
That's just my idea. Let me know if you agree that his ult should change or not and feel free to drop some ideas. The title of this post should probably say "Why Veiger's ult should/could change, but the important thing is that something happens to boost Veigar up from the slump he's in now, it doesn't necessarily have to be his ult.