Right now, Rubick's new passive says that it increases duration of debuffs.

This is too general. The duration increase works like status resistance, so it does not affect every debuff. For example, purely damaging debuffs (like Poison Nova, Firestorm, Macropyre, etc) are not affected by Arcane Supremecy's duration increase.

Damaging and slowing debuffs (e.g. Dual Breath, Fire Spirits, Icarus Dive, Liquid Fire, Flux, Psionic Trap, etc) have their slow values increased (because status res reduces their values, Rubick's passive increases it).

So instead of saying "DEBUFF AMPLIFICATION", it should say something like "STATUS AMPLIFICATION".