Some possible suggestions for amumu in the vanguard update.

I just want to say that is my Favorite tank both gameplay wise and thematically. I'm also under the impression that he only need small scope changes, being that only his Passive needs to be changed entirely and his W needs an extra effect to emphasize his theme of "stay the hell away from this champion" more.

My suggestion for his W is to Add an effect that reduces opponents MR by 1 per second and that scales up to maybe 2 or 3 MR reduction per second at max rank of the skill. This effect doesn't stop until the opponents MR are all reduces to 0. So basically the longer you stay near amu, the more screwed over you are going to get, and I think it works pretty well with his theme.

As for his passive I think it should be entirely changed, with the MR reduction being on the W passive effect I just mentioned. Atm I'm not sure what that can be, but it should involve something happening when 5 champions are around at once. Maybe some kind of strong curse effect that only triggers with 5 champions are around him.

Just my 2 cents, Others can throw in their suggestions as well. I really don't think he needs huge changes other than a complete rework of his passive.