Vel'Koz changes why they make him weaker.
The reason Vel'Koz's kit is fine as it is that Vel'Koz has clear defined weakness that can be over comed by smart positioning and playing it safe earlier game by farming. In this meta of top picks mid are some of Vel'Koz hardest counters being Zed, Ahri , Talon and Fizz. It really helps to have your W to safely farm your lane without the constant fear of being all ined at any moment.
Passive Scaling-I don't see why it needed a buff it is pretty good as it is. If your going to mess up his ult you need to increase the stacks duration by levels.
Q Change- It is fine and much needed since Vel'Koz is a mana hog.
Why the W change is awful. You are weakening his W because it is a mindless farming tool? That is stupid then I think it is mindless that Uydr can sprint in Bear stance forever and Blitzcrank gets tired after a brisk jog. Your SPICE things up is just going to cripple Vel'Koz's farming ability as a mage with no escapes going into the middle of the lane is a death sentence since top junglers will turn your arse inside out.
E Change- Why even nerf it? Atleast give it a wider hit box or faster projectile speed. An unnessary nerf to his only escape. Maybe shorten the cooldown
Why R change is a stupid showy gimick- His Ult is now so shoe horned into his kit that completely removes his teamfight threat except for the tank. His ult's big utility wasn't as a big hurrah death beam but as a mean to help clean up after fights. I cvan't tell you how many teamfights were won by blowing up my passive at the end of it. That extra true damage from blowing up a stack has ended more ADCs then I can count. This research thing is a stupid Gimick that in THEORY looks cool and devestating to use but it won't work out the way Riot hopes.
My Closing thoughts
I love Vel'Koz since his release played him everywhere even top and carried myself with him. I even have him as my background I know Riot won't allow him to stay as he. I fear he will be the Gangplank of mages after this a weird Re-re-re-rework waiting to happen. I really really hope they pull the plug on his rework. I do not want my favorite champion to get ruined.