My opinions on changes to Karthus that would make him more viable

Increase passive timer at higher levels increase passive ability effects - maybe like lower cast time of all spells or increased CDR while in passive

Defile larger AOE or more damage per second> tick rate like rumble q wall larger/half a second stun or increased slow q more damage, farther casting range or increased radius Ult more damage - increase pre-existing cc effects like rylais Maybe include a little DoT while he is casting ult?

Karthus not viable in meta-never will be loses all matchups not reward feeding but help the user feel less punished/more valuable