Hello, in case Riot will be thinking about reworking Amumu a tiny bit, I've had a couple of ideas to give out.
His W could be a spell that increases the damage done in a small specific area (as big as a Viktor's W or smaller) while at the same time shortening the range of spells and autoattacks for both enemy and allies.
His E could be the opposite, increasing the range of everything coming out from a small area but decreasing the damage.
His W/E AOE damage could put into passives (into his basic passive or into passives put on the spells), like he does AOE damage every auto and/or every spell.
These new abilites would sinergize well with Q, and with R, and would have really interesting interactions with allies/enemies, giving higher skill cap while not over complicating the kit. Otherwise, you can use this ideas for some other champion.
I will be quoting this when Riot will implement similar ideas in the future (like they often have done in the past) :)