I'd like for them to keep his design mostly similar, but update it and tweak a few things. Veigar's model is very old, from back in 2009 when League was still in beta and Riot was a small indie company (no kappa). A lot has changed since then, and given the love he so rightfully deserves, Veigar could look amazing ingame. I understand Riot can't please everybody but I do think making a thread like this is a good step towards better communication between Riot's VU team(s) and the players. It is only natural that everyone has a different vision, but trying to find common denominators amongst the people who play and love these champions should give Riot some sort of idea for what they should give priority when redesigning their beloved characters.
The most important aspects to keep to me are his iconic look, personality and goal as a yordle in the world of Runeterra.
Veigar's story and personality
His traumatic past twisted him into something sinister and sadistic, the kind of 'villain' the world could use. Veigar's goal is of course to forcefully unify the world and become supreme-ruler of the world. He simply wishes for peace, but he is willing to go to extreme measures to attain it. He covers his true self both metaphorically and physically, doing everything that he can to convince people that he is evil incarnate. No one besides Veigar knows his true intentions, but to me he has always been an anti-hero. His ways may be evil, but his goal is pure.
Veigar on the Rift
Veigar has an extreme height-complex, it is unclear if he is short even for a yordle but I think that is irrelevant because it is not to yordles he is comparing himself. To Veigar, the small stature of his species is a big problem. Let's be frank, yordles are terrifying creatures, but their outward appearance does not reflect this at all. They're tiny, fluffy and generally considered "cute", and as such I'd imagine the people of Runeterra would look down on them, in both senses of the word. As an evil overlord Veigar demands respect but is never taken seriously. His majestically squeaky voice isn't helping things either. Even for a yordle people might think he looks weak and pitiful, trying too hard to be something he's not and instead coming off as the exact opposite. Veigar dons his iconic pointy blue wizard-hat that serves a double purpose. Not only does it make him appear taller than he is (although the top is too heavy to carry its own weight and instead flops backwards.) but it also covers his face in a shroud of mystery with those glowing yellow eyes peering out. Veigar always had a sense of mystery to him, and this has always been a big part of his appeal to me. Veigar is entirely covered from head to toe, leaving much to the imagination. His robes and metal gauntlets have a certain appeal, but aren't completely necessary to me, the most important part is his aura of mystery surrounding him and his attempts to hide his yordle-like appearance from the world.
Voice lines and interactions
As much as I love Veigar's current voicelines and find them all iconic, there's no denying that today's champions ship with at least 10 times the voicelines, and hearing the same quotes over and over may annoy some players, and adding more lines could help flesh out Veigar as a character further. Interactions with some of the other inhabitants of Runeterra would be great too, to further bridge the gaps between the characters and make the world come to life a bit more. I'd especially like to hear more about his time as a wandering apprentice, his future plans for world domination and his relation to Lulu. Veigar and Lulu have many things in common, both being outcasts of Bandle City and having spent much of their lives in solitude, both being talented magicians and both being more or less insane. Lulu was suggested to be a friend of Veigar years ago, but we have not heard anything since then about their bond and it is a story worth delving deeper into.
Final thoughts
I think Riot did an amazing job with Veigar's new splash art. Most players may not have noticed it, but Veigar did actually get a minor overhaul in this splash art. His robe was redesigned and looks far less like a clunky tutu and more like a black-mage robe, and I believe that simply trying to replicate this design in-game would yield the best result. It has the appeal of the original design that we've grown to know and love, but with more flow and thought behind it. Making a new model following the splash-art as a guide-line and updating Veigar's proportions to match those of the new/reworked yordles could turn out fantastic. If possible, making his eyes appear to be burning like in the splash would be amazing too, but it's just a nice touch and not necessary in my opinion. Something I would love to see brought back are the runes on Veigar's staff. Ever since his texture update that removed the runes, his staff has appeared far less like a magical sceptre and more like a toy halberd with a crystal on top. Veigar is, despite what the ignorant people of Runeterra might expect, one of the greatest wizards in all the lands after all, and his staff should reflect his power over the Black Magics.
Thanks for reading, and lets hope our Tiny Master of Evil will become even more Superb in the near future! 
"It takes a lot of effort to move like this in heels" 
"So many noobs. Will matchmaking ever find true balance?" 
The Swift Scout vs The Tiny Master of Evil
It seems every region has their great rivalry.
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And I believe these belong here too:
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Im of the sad.
