Only problem Darius has ever really had is that he's a noob stomper, but his Q being designed the way it is offers a lot of interaction between players, which is good.
I think Darius is solid early and mid game, but can get outclassed late game, which is okay imo. Since he procs CotC so reliably, he can get a nice shield which has enhanced him at all stages of the game, especially late since it scales really well, and since that's the area where Darius can struggle, you have a winning scenario.
If he needs to be buffed, buff passive damage.
If he needs to be nerfed, raise mana cost on Q.
I don't feel like he needs a lot of work, but I have my on quips about Q's power budget.