Nunu, selfish Tank? Let me stop you right there... First of all, Tanks aren't selfish by nature, since they have not enough damage to kill an opponent alone. Otherwise, they'd be Juggernaut, which you just said shouldn't be Nunu's role. But if you want to be selfish, then again, Grasp is a perfectly good Keystone. Remember the damage proc is twice as strong as the heal, so no, it does much more than just "help you not die".
But more specifically, Nunu, selfish Tank? The guy who has a 2-person AS buff, whose current best and most common build (by champion.gg) start with Cinderhulk into Knight's Vow into Solari, and who Meddler mentioned sometimes take Stoneborn Pact and Ardent Censer in the current meta? You should check your definition of "selfish". Guardian totally fits the current most common Nunu build.
But OK, let's admit you don't want Guardian or Grasp, for whatever reason. You don't like them. OK. Then I propose to you this page: http://na.leagueoflegends.com/en/featured/preseason-update/8200-8214?build=20122226
Aerie should be deceptively strong: you can get a CD refund by walking to your target and picking up the pet. But since you're melee, that's almost instant, so you get the proc on every single snowball. In a pinch, you can also use you AS buff to give a shield to your carry. The tooltip also doesn't say "champion". Might just be a mistake, but it might be that it works against Monsters too. Anyway, even if it doesn't, that's still a super cool Keystone, that works pretty well with your kit.
Then you can opt for more Mana Sustain or more CDR on your Ultimate. None is very exciting, but both are generically useful. Or you can opt for the defensive option if they have an heavy AP team, but that's much more specific.
Below that, though, you have Absolute Focus. Free damage stats when you're at high life percents, so works pretty well to increase your clear speed, especially since Nunu high sustain in the Jungle helps him keep it active at all time. Alternatively, if you're happy with your base clear speed, you can opt for more CDR or more MS to help your ganks. All in all a pretty great slot.
Finally in the last Slot, I'd pick Gathering Storm, which basically puts a ticking bomb on the enemy team. 'cause if the game ever goes past 40 min, you will just wipe them all with your R. But if you don't like to drag out games, Scorch is a fine proc for ganks, and the MS+AP in the River is a bit specific, but still pretty nice for a counter-jungling champ like Nunu that spends a lot of time in the River.
For your Secondary, you splash into Resolve to still get some durability. Font of Life gives you permanent healing on-hit on the target you snowball, and you can proc it team-wide with your R. All your allies benefit from it too, and it's in % of your max Health.
Which combo great with Overgrowth, an infinitely stacking HP buff, that also work in % of your maxHP and gives you huge late game durability.
Overall, this is a great page for Nunu. You have free heals, huge late game damage, huge late game durability, faster clears in early game, additional damage on your spammable snowball and some bonus utility you can use in a pinch. Over the 6 slots, 5 of them have great synergies with your kit as a whole, and the last one is still pretty generic power that's never bad. If you don't like late game builds as much as I do, you have perfectly fine alternative to replace Gathering Storm and Overgrowth, including one of the few Minor Runes that screams "made for Junglers".
(But why do I have to find a good page for YOUR champ? Why didn't you do that yourself already? I mean, the website is out there for everyone to use... Go explore a bit, for fuck's sake!)
My point is: why are you complaining? You have perfectly good tools to use, and you're refusing to use them, and asking for a worse tool instead, just because you don't understand that you are NOT wasting your Keystone if you can't use it on Monsters. I repeat, you're not using your Ultimate on Monsters either. Do you think you're wasting your ultimate compared to laners who can use it on their lane opponent whenever they want? Of course not...
You seem to thing Strength of the Age was a good thing, but it wasn't. All the best Junglers at this time were Tanks that could abuse it, and even champs like Elise were forced to take it, because they were outclassed otherwise. You shouldn't want to be forced into a meta-warping Keystone, it's much better to have choice and flexibility so that you can pick whatever champ you actually want and then pick a Keystone that really suits them. Not the generic Jungle Keystone that everyone is forced to take.
). So is 3% extra tenacity for each 8 large monsters (one full jungle clear) worth? ...Hm. I guess it's not terrible.
and an assassin 
for that.
