**"No." **
That was the first thought that came to mind after reading your proposal. The whole point of riot nerfing Ryze's passive stack duration was because he wasn't delivering on their idea of "windows of power". Essentially, you were waiting a REALLY LONG TIME for his stacks to run out just to engage on him.
If he were allowed to activate his passive on-demand, he would NEVER be weak.
Everybody knows he is a strong duelist, with the ability to catch AND kill anybody he snares with his passive ready. That's why they made it more difficult to keep his passive primed for activation; It was too easy to abuse. Now Ryze players need to master the new stack duration of 6s, and imo, it is COMPLETELY FAIR. It's long enough for you to use q every time it's off cd to keep up your stacks, and short enough so that melee champions aren't completely cut off from cs or even from trying to fight you.
Most importantly: At lvl 16, his passive will be PERMANENTLY activated in combat.
If you manage to combo correctly after rank 3 ulti, you will notice that your ulti comes off cd DURING your passive's duration. This means after 3-4s of spamming abilities, your ulti is ready to be activated again to continue the onslaught of spells. The last thing anybody needs is a champion spewing out ranged AoE abilities with no limits except for mana while being tanky at the same time.