We have three 20% CDR/AP items
One of them is super-niche and of the other two, only one of them is ever worth building because they have too much overlap.
Nashor's should have a blasting wand added to its build and its passive made more attractive to a wider range of Mages to cement it as the GO-TO offensive CDR item. 
As for Morello's and Athene's, they're both giving AP/CDR/MP5. Way too much overlap right there. I would suggest sticking Idol into Athene's build and increasing its MP5 substantially. While we're at it, Chalice gets its build altered to a Fairy Champ and an Amplifying Tome. Massive mana regen combined with more easily shrugging off harass leads to bland farm lanes and I don't think anyone really wants to watch that. Removing defense from Athene's allows its purpose to be better defined. 
Then? Replace Idol in Morello's with Kindlegem and toss in a Giant's Belt. Replace the Grievous Wounds passive with a defensive passive of some kind. Small damage shield on spell-cast, maybe? A sort of anti-Spellblade. This would be widely useful on tanky Mages as well as Utility Tanks without being terribly attractive to AD Fighters on account of the ability power. 
Offensive item, mana sustain item, defensive item. Clear niche for each of them. Riot plz.
), viable on others ( 
), and it's literally the only decent AP/AS item unless you want to go full triforce, which many champions do not.
... a largely inferior (but stacking) on-hit effect, also 50% AS, but nothing useful if you scale on AP.
when i think i can handle/evade the magic damage or my opponent.
if i know i can not, i build 
.
Those are two completely different scenarios, Two completely different games.
Stop wanting to streamline.
There is no "one build item fits all situations" path