I thought about something like this:
If Noxian Guillotinne kills the target, it's cooldown is refreshed. This ability can't be used on a last champion it killed for a time equal to 100/50/0% of its cooldown.
How does it exactly work?
-Darius' level 1 ultimate has a 120 second cooldown. He uses it to kill his lane opponent and it resets - but he can't cast it on said opponent for 120 seconds. In other words, it will be available to use on other people.
-Only 1 target can be marked at the time - the last person killed. So he kills his lane opponent and marks him for 120 seconds. Dragon fight erupts, he TP's down and gets a kill on their ADC. Now ADC is marked and he can use his ultimate to kill his lane opponent again.
What does it mean?
At level 6 Darius can dunk his lane opponent and will have his ultimate for skirmishes, dragon fights or an enemy jungle gank. But he can't dunk his lane opponent again until the normal ability cooldown runs out.
At level 11 it's similiar as level 6, only he can dunk his last target much sooner.
At level 16 he gets his full reset back.
Purpose of these changes is to give him most of his reset power back while still removing the problem of murdering your opponent every time he shows his face in the lane.
And yes, Darius should be able to completely murder someone. Reliably and often if he gets into range. We could use more immobile champions seeing play.