As an Amumu Player...
Amumu has been one of my favorite champions in the game. He can completely change the game in just a few moments and I enjoy those moments every time they come. I also feel like he is in a great spot, and it seems that Riot agrees. His windows of power are clear; get CDR and be a monster in teamfights. Just as he has windows of power, he has weaknesses as well. Invade Amumu early and force duels to shut the mummy down. As someone who has played a LOT of the Sad Mummy, I felt called to share some of what I would love to see.
First: New ability icons They're fairly old and could use a face-lift.
Second: New effects on Curse of the Sad Mummy (ult) The ring of emoticons around it just seem...in need of a visual upgrade.
Third: Give a tiny bit of love to Tantrum's passive I'm not saying that he needs it, but it would be welcomed. Make the 2/4/6/8/10 physical damage reduction double against champions (4/8/12/16/20 physical damage reduction from champions). It softens his early game (vs invades) a little bit and would have a noticeable effect late-game.
All and all I love Amumu. Like I said, he is currently in a great spot. The Sad Mummy's kit makes sense lore-wise (he needs friends to be effective), and I find him to be a great solo queue champion. Thank you Riot for keeping him in the spot that he is.