Nunu has AP scaling on all his abilities, and yet, almost nobody ever plays AP Nunu.
One thing that might help making AP Nunu more viable, would be, if Riot fixed his passive.
Nunu deals some splash damage with his autos. This splash damage is currently "30% AD". It's not splashing continually though, it's only occasional. It's the only AD scaling in his entire kit. Nobody builds any AD on Nunu, so this splash is just a tiny bit of damage coming from his base AD, mostly used just to help apply Hunter's Talisman to the little raptors, and that's about all it is good for.
The reason AP Nunu isn't viable is because, without Cinderhulk, Nunu's clears just aren't good enough.
If Riot was to switch this 30% AD splash to some percentage of AP instead (maybe giving it also some flat damage just for early game when Nunu has 0 AP), Nunu could use that AP splash damage to help clear the little raptors with an AP build, and skip the cinderhulk entirely. That would make AP Nunu viable. Riot might have to adjust some of his other AP scalings though, they are quite generous on his other abilities, and if his clears as an AP were decent, those high scalings on his other abilities might end up being too good actually.