Lol the Nunu pentakills are coming

It won't increase the odds of a penta much more than what's currently available. The damage of the ult isn't the problem. It's the fact that wards, the slow that you gain upon touching the aoe, and having five people walk into it all work against the odds of it happening in the first place.

The chances of having an Amumu, Galio, and Nunu all in the same game, or a group of champions that will help Nunu get a 5 man ulti are usually pretty rare. A Ryze/Nunu combo would be more likely, but even then probably hard to pull off.

The only way I see nunu becoming some pentakill god is if the team comp is completely wonky and the standard nunu build changes from a tanky build to a standard mage build.