We'd go back to the original idea. I should dev log Darius' development sometime.
Originally, Darius had 7 stacks of hemorrhage and the ramp-up on how much the ult did was much more stark (low stacks were terrible, full stacks was what you see now - and it only did true damage at max). This wasn't perfect, but it's much closer to the design intent; let me explain that and then you'll know where I'd like to go with him in the future.
For Darius, his gameplay should be about building stacks and keeping them up over a longer period of time. When he reaches a threshhold that's actually difficult to reach, then his ultimate should rip you in half. I'd also like an ability for him to spread stacks for real insanity with this design. Let me give you my idea as it stands today;
The play pattern is that Darius wants to get to 10 stacks to just murder people hard. The counter-play should be to get him off you at least long enough for stacks to fall off. The longer the fight, the more advantageous to Darius. Ratios and stats changed to favor lower AD, tankier build to support this.
Hemmorhage: Changed to maximum 10 stacks on a target. DoT damage adjusted to match, with something like 5 now = 7 in this model. All skills interact with # of hemmorhage stacks on the target. Auto-attacks and skills apply 1 stack.
Blood-Soaked Blade A slight redo on the Q, this still does PBAOE damage and adds a stack of bleed. Any targets hit have their hemmhorage stacks shared with that target's allies.
Crippling Strike No longer gets shorter cooldown with hemmorhage, but instead increases the slow % and duration per stack. At max, this would get to something like 80% for 5 seconds. At 0 stacks, 0% slow, but still an AA reset.
Apprehend Unsure, but maybe just numbers.
Noxian Guillotine Does (small) base damage, +(OK) AD ratio. Base damage and armor penetration (10% per stack) increase per stack of Hemmorage. Ult STILL RESETS, as ten stacks should not be a particularly common occurance, and should be the state Darius is trying to achieve (and the state enemies are trying to prevent).
So, this would require testing etc, but it captures the spirit of Darius that matches the original intent of something that significantly ramps over time, and the gameplay revolves around preventing Darius from reaching short-term critical mass in an engagement, while he's trying to set up limited windows for a kill. The psychology should exemplify tension, as every move he can just hold on to for a little longer has greater power when used - either getting greedy by waiting too long or getting zealous by using things too early will cause Darius to miss his chance to beat someone in a fight.