I haven’t seen any replies yet about his pro play, but I have a few thoughts on him.

• AoE stun - not much else to say. This was enough to put veigar into the spotlight as a support. It’s entirely possible that someone decides to play him as a roaming support just to burn a flash lv 1/2. Also great for counterengages and zone controls. It’s not a tremendous cooldown either. Is the rest of his kit useable enough as a support? Time will tell. But as far as surprise picks, don’t count it out.
• Splitpushing - his ult gives him some great utility. He can splitpush, then recall and homeguard into a fight with TP. He can splitpush and avoid getting picked off by a rotation. He can buy items and return to splitpushing with his ult. He can TP, AoE clear a wave, and ult back to the team. Not all of these are as effective as each other, but he’s got a great way to mindgame / outrotate the enemy team. Or, just repair a bad rotation.
  
• Seiging - this is where I think he’ll shine. Seige comps love champs that can easily waveclear, get tower damage, or just rotate quickly. He can do all 3. He can burst down a wave with a damage combo. He can hit the tower with a lich proc and ult out (or just threaten to ult out) if he takes a lot of poke. He can engage and burst down the front line and ult back to avoid the rest of the tower aggro. He can rotate to a nearby lane back and forth and just ult to be with his team if he needs to return. He can do this faster than the champion who is stuck covering him when he moves.
  
• Roams/Ganks - he has a good gapcloser to get close, and a stun to chain CC. He can towerdive and back out even if the looming tower hit would normally finish him off. And there isn’t a whole lot of counterplay to it.
• Being dove - he can handle 2v1 lanes or early towerdives pretty well. His AoE stun around himself is a great deterrent to hitting his face under a tower. He can heal damage and teleport with his ult. He can get a giant shield. He can dash away from the dive and blink to the other player for a second dash if the positioning is a better fit. If he survives, he can warp right back to lane and miss no CS.
• For an assassin, he has a very forgiving moveset for being out of position.
• His chase potential is very high.
• He can work great in a catch comp.
• He can work great in a peel comp.
• He can work great in a disengage comp.
• He can work great in a seige comp.
• He can work great in a splitpush comp.

Overall, I would be surprised if we don’t see him in professional play, which, if we’re honest, is the real reason he will receive nerfs in the future. I think his kit just makes him a very versatile threat. I really think you will be able to tell a difference between pros and their play styles just by how they choose to use him. He’s a jack of many trades. Only time will tell if he’s a master of them too.