You realise that his mana pool is quite small to begin with. You basically want him to use no other skill for him to even attempt ulti chains (because you know using Decimate, Apprehend and Crippling Strike doesn't take mana). If you want the increasing mana cost on his ulti by 100 per use, then give him something to make his other skills viable. Say, all other moves have their mana costs halfed and are 50% more effective then what they are currently.
For someone who is supposed to be in the fray of melee combat, he lacks innate tankiness and/or sustain to get to the fray, and he lacks any form of true gap closer to actually give him the boost he needs to get into the fray. He has issues that the other bruisers lack. Leave his ulti, since it's the only thing that makes him decent to use.
He could use a hugely increased scaling on his Q. His Q starts to get really weak once the game reaches a certain point. An increasing mana cost on his ult shouldn't be much of a problem. It would just stop him from being tempted to spam his ult for every single kill, which is a big reason why people don't like having him on their team. What happens really often is it's not even necessary in the slightest for Darius to ult but he uses it anyway. If he had some damage put into his other abilities, he could still contribute the same to teamfights without having to "secure" kills on people that would've died anyway.