Nunu has a change on the PBE that might actually make him good
Nunu has been a meme of a champion for most of the preseason, simply because there doesn't seem to be a good keystone rune to go along with him. He's weak overall, too, and without combat effectiveness and a decent rune, he just feels terrible to play.
To fix all of that, Riot's balance team has pushed some changes onto the PBE for testing. They aren't changing a rune to match his playstyle, but they do buff the hell out of his kit. They'll improve him so much that won't just be viable—he might actually be good.
Both changes are brand new features to his kit, not just stat changes or buffs. The first will grant him magic damage for each basic attack after becoming Well Fed through Consume, and the on-hit damage scales with five percent of his AP. This will make his basic attacks much scarier, improving his ganking and his clear speed.
The second and most ludicrous change is coming for his Blood Boil. After casting it, both he and the ally he casts it on will gain a 40 percent boost (depending on level) to maximum AP on top of the attack speed buff it already provides. This means that it can bolster your AP more than a Rabadon's Deathcap. The boost has a cap, though, and will only provide 40 to 120 AP depending on rank.
This massive buff will not only make mage or mage-bruiser Nunu a possibility, but it opens up more options for him in other roles, too, such as mage-tank support with an AP carry rather than a marksman bot.
These changes are up on the PBE for testing right now, so they should go live in about two weeks if they don't get reverted or changed beforehand.
Correction Jan. 12 10:30am ET: Fixed an error related to the Blood Boil buff.