Darius is a lane bully that typically dominates in lane and falls off hard in mid-late game team fights. He is not particularly in need of buffs for anything sub challenger according to u.gg , but I suggest a power shift / gameplay change to how Darius Q scales with CDR.
Darius Q at the moment does not start it's cooldown until the end of his Q animation, making increasing CDR having *diminishing returns*. Darius could have 100% CDR and his Q would still be locked at an effective 0.75 sec cooldown due to the cooldown starting at the end of the animation.
At the moment Darius Q is the main ability for Darius skill expression. Players feel rewarded for dodging the blade portion and Darius players feel rewarded for landing "sweetspot" Q's. I feel gameplay should be emphasized more around his sweetspot / juggernaut tanking mechanic , but CDR builds (BC + spirita visage) are held back by his Q cooldown always having an additional 0.75 second cooldown added at the end.
I think a change to how his Q cooldown begins would create more rewarding skill expression for Darius players, and would be fine as long as power is taken somewhere else (perhaps base damage on W or armor penetration on E). This will also remove the raw damage aspect of Darius that he has in lane , allow him to transition better as a tanky juggernaut that wins in prolonged fights.
Tl;Dr Remove power from W and E and allow Q cooldown to start at the beginning of cast to put more emphasis on skill expression / reward of Q.