Id like to put in a suggestion for Karthus's Ult

Instead of damaging all enemies, only damage enemies who have been visible in the last 15 seconds or so. This is so that it can be used to kill the low laners who have recently fled from vision, or a jungler after a gank. But it stops him from ulting for the enemy laner(s) and then get a kill the low jungler by having a camp finish him off. This isn't fair. OR even when someone leave so the match early and he can one shot them while they're in base since he's so far ahead of them. This constantly happens and always feeds Karthus.