Since Nunu will now bloodboil AP..
Maybe it's time to scrap Absolute Zero, make his ultimate Blood boil, increase the duration and power and add damage mitigation when Empowered?
Then they can actually give him another damage or CC ability instead of just snowball?
I'd go something along the lines of this.
Yeti charge. (remember he's losing a lot of mobility with bloodboil going ult in this proposal) Willump lunges forward a short distance colliding with minions knocks them up whereas colliding with champions knocks them back, champions collided with will take extra damage if they collide with other minions or champions. If Empowered, Willump will instead grab the enemy champion and toss them a greater distance enemy champions they collide with will be also knocked back a short distance.
If there was to be a full rework however I'd look to make Willump a Grappler and Nunu a Savage like Ferra Torr from MK X.