Like I said, it's a strong build. There really isn't a ton to change. If there are any thoughts I could offer they would mainly revolve around adding just a little more life gain. Kalitas'
Lifelink is good and all, but he is a massive target for removal. What's more is that I played against a deck a little while ago that ran heavily on Llanwar
Wastes (the green/black version of Cave) and they took a pretty heavy toll on his life total over the course of the game. So, maybe add or at least sideboard something like
Tenacity or
Ondu Rising. You could drop Self-Inflicted Wounds for that.
Another thought is to
squeeze another Displacer in there if you can, maybe at the cost of one of the Stranglers. Blinking a Seer repeatedly is a fantastic way to control your opponent's hand. Maybe help mitigate their removal of your things or counters to them. It can also protect your creatures from destruction. Basically it is a very powerful ability that is at your disposal. An extra would increase the likelihood of drawing one.
Lastly, maybe a little ramp. If nothing else, than in the sideboard.
Hedron Archive is a personal favorite as it drops for four mana and taps for two. It's amazing what can be done with seven mana on turn five. Other options though, could be
Kozilek's Channeler or
Sifter Of Skulls. Both are Eldrazi. Both work.
Those are some thoughts. The one I would feel the strongest about though is the life gain one. The guy I played couldn't get all his colors out and had to rely on those lands, which hurt him a lot. I almost felt bad because I really beat the crap out of him for it...