They aren't working Vel'koz because he's imbalanced, or weak, or OP. No, they are reworking him because he simply isn't unique. Mages like Xerath/Lux/Ziggs/Vel'koz are all similar.
They want to make Vel'koz unique. Let's just wait for the rework or give some suggestions other than "Don't rework him"
Saying don't rework him isn't going to solve anything by how Riot works lol. Better just to say what you'd like to see about him to make him unique. I personally want to see a knowledge type mechanic involved with him in some way.
Update =/= Rework please understand this part.
But yes, they want to really drive home his fantasy and make his reason to be picked over other types of immobile mages good.
Sometimes it's about balance: 
Can we do something about the dime-a-dozen top laners who have highly similar kits? Gap closer. Auto--attack enhancement/modification. Some form of stat steroid (probably attack speed or bonus damage). High damage Ultimate. If that sounds familiar, that's because it's the basic formula for a lot of top laners.
I think a better way of phrasing it is rework are designed to eliminate certain problems that Champions have, usually its regarding the way they play or their roles.
Karma is a primary example of a mage/support whose role wasn't niche enough, she didn't have the range and safety of an AP Mid. Shen is an example of gameplay, where they saw unique ways to make his laning more interactive, by gutting his sustain and giving him shields of course...
Well.. it's true, sometimes its not about balance or power, but that they don't live up to the current meta or trend.
