Okay, extra CDR would be painful to balance, but take a look at this:
Blitz has 700 BASE DAMAGE with QER at level 6, that's just ridiculous. And it's before runes/masteries/thunderlords! By gutting (or completely removing damage from some abilities) they could get much less "feast or famine".
If Blitz misses hook he's useless for 20 seconds. If he hits hook its guaranteed kill mostly due to his damage (especially in laning phase).
What if his hook had 50% of its CD refunded if he fails to grab something, but his grab doesn't do damage anymore? What if his ult dealt 0 damage to champions but had 0.5/1/1.5s GROUNDED instead? It kinda makes sense, static field magnetizing you to earth, making you unable to lift off for a while.
As for Janna - yes. Her current shield is 240+0.7AP with 10s CD. If we made it 200+1.5AP with 30 second cooldown, wouldn't it be much healthier? This way Janna has to think before using shield, as its long CD creates window for enemy counterplay. To compensate, it's ridiculously powerful and it scales with AP instead of just having high bases, which means Janna goes either full AP (gets blown up easily) or supportish/tanky (weaker shields). If she needed some way to decrease her CD on shield, she could always get a mechanic that decreases E's cooldown by doing something offensive to enforce active play
Leona is pretty healthy atm, but it would be lovely if we moved more of her up-front damage to her passive.
Thresh could get UTILITY scaling with his souls, something like: W refunds CD and mana for every soul gathered by it, decrease base shield, improve bonus shield from souls (2-3x more?). Also, imagine this "If Thresh gathers X souls he weaves net out of them. His Q is now cone (or ground targeted AOE), pulling ALL ENEMIES HIT towards him. He can reactivate it to pull towards enemy champions hit.