The problem with counter jungle Nunu is that he can't duel early all that well so he basically relies on stealing everything from the enemy Jungler without them just walking into his jungle and taking his stuff. Basically, in order for Nunu counter jungle to work, you need a full team dedicated to defending your Jungle. However, even then, Nunu's clear speed for early camps isn't quick enough for him to raid the enemy Jungle and get out with a net gain of gold.
If we honestly want Nunu to go back to his super counterjungle days, his Q would need to deal incredible amounts of more damage (something like 700/850/1000/1150/1300 from the current 400/550/700/850/1000) and have it's cool down reduced heavily (something like 9/8.5/8/7.5/7 seconds from the current 13/12/11/10/9 seconds). The main question is how much would this break the epic monster system?