TLDR: go to the "Buff OR Quality of Life" section I am suggesting.
I have played a lot of Zilean since his rework and really enjoy everything about his kit (especially the Q attachment logic change, that was brilliant!)
The recent change to his passive (at least 15% of exp stored up) was a step in the right direction, but it still feels like a gamble to level someone up.
The reason I say "gamble" is because if I haven't seen my ally's circle recently turn gold, I cannot be sure how much of my stored up experience I will give them (even if I know the minimum is 15%, I still feel like I've lost the "gamble" when I see only 75 of my 500 stored up exp go). I feel like I have to be constantly paying attention to my team-mates' exp in order to know how worthwhile to the team it will be to level them up - this kind of secret knowledge feels like exactly the kind of thing you've been recently removing from the game (e.g. we all see Dragon's/Baron's re-spawn timer, our allies' ultimate timers and their summoner spells, where as before it was "secret knowledge" that only people who paid super close attention to, or are on Skype with each other would know about).
I think this "gamble" could be removed in two ways, one of which would be a buff, the other a quality of life change:
Buff suggestion: For each % of the stored experience NOT used up, the cool down is reduced by a certain percent. This ratio could be tweaked to balance the buff. For example if I had 500 experience stored up, but only used 200 then I would have not used 60%, and the cool down could be reduced by say 30% (I.E. a 1 to 2 ratio) to relieve the pain of losing the gamble.
>>>Quality of Life Suggestion<<<
Quality of life: Change the circle to not be golden, but simply a 3 tier colour system of green, amber and red (or more than 3 tiers, who knows) where green means you'll use up 66% or more of your stored up exp, amber means between 33-66%, and red means less than 33%. This way if I see two allies who I both think deserve my exp (e.g. they are both snowballing and I want to make both of them stronger) but one is red and one is green, I know I am helping my team more by choosing the green one instead of guessing (or gambling).