Since Protea's kit is already very good, it's understandable that her 4 have a niche support use rather than having damage be the priority, but also I can't help but feel like since it's her 4 it should have more... oomph. So how about we rework it? I have a very simple solution for this as well, it would involve just changing the way the damage discharge works.
So, you know when the ability is active, it creates after-images, right? How about once the timer ends we make the explosion affect every single spot the afterimages are created? It would sort of work like the 4th ability version of firewalker that way, but instead of lingering damage, it's one big burst of damage at the end. This could incentivize building for duration and strategic positioning, amongst other things.