Add Runeglaive and KEEP Magus
On the PBE right now there is a new jungling item enchantment that replaces magus called runeglaive. It provides an on-hit effect and builds out of sheen. I know the PBE is tentative, but I'd like to provide my own personal feedback and see if others agree or disagree.
Some champions don't benefit from the on hit effect, like
or pure AP
, so...
...why not just keep magus in additional to adding runeglaive? Is 5 jungling item enchantments too many? I personally don't see a problem with having 5 instead of 4.
Runeglaive isn't a flat out buff to all AP junglers. Replacing the niche magus junglers with runeglaive junglers does increase champion diversity in the jungle, but keeping magus and adding runeglaive would increase champion diversity moreso.
over 
but every other jungle ap champ is either someone people take to the jungle for fun
or an off build that is not currently intended
.
or
, then you're all like "shit unique passives."
is enough for Amumu in the mid and late game. Since you can't get runeglaive until mid game anyway, it won't help resource management too much.