There's a simple way to make Veigar more Veigar-y

Instead of just removing the 80% enemy AP ration, which currently doesn't fit into the champion that well, and then simply beefing up the ultimate's numbers, perhaps consider a slightly altered approach. Keep the same AP ratio the ultimate has now, remove the enemy AP ration, and add in a ratio that give his ultimate more power based off of the amount of stacks he has farmed with Baleful Strike.

Rather than just making his ult more reliant on big AP items, it actually rewards him for utilizing his kit well. Also, gaining an advantage over the enemy through say, a bunch of jungler ganks that resulted in kills, does not as directly correlate into more stacks as it would into AP stats so there's less of a reason to worry about snowballing than there would be with a straight up ratio increase.

This change would no longer favor Veigar's power in the direction of AP mages for no reason, but it would also give him so tools to both gain and keep and advantage over his enemies. Veigar is the tiny mastermind of evil who will blow up everything you know and love if you leave him to his schemes, his ult shouldn't randomly say "or if you build AP, cause other mages suck! Or something, I guess..."