to be honest, I would do the same thing I'm doing in this world "study genetics" and science in general - my goal is to stop the aging process, So I guess I'd be orochimaros best friend until I gather all his knowledge - then maybe I'll have that bastard killed.
I would study to the very depth of medicine - learn from Kabuto, Tsunade, and all their best medics and Scientists, when I am immortal then I Would start my own experiments in the shadows and keep going at it for thousands of years - I will rebuild my body/mind to the very peaks of perfection and when I'm ready I will build an army of my choice robots/humans/beasts/mutants whichever I find to be the strongest and I will conquer the world without a single pause.
Once I have conquered the world - I will reform humanity, Overwrite their genome "partly" - either remove harmful instincts/intents/emotions or give them complete control/balance that will lie within the depth of their cells "irreversable by them".
when I have conquered, reformed and balanced the world - I will go back to my science and I will take millions of scientists with me, give them immortality and have them work on different experiments "needed by me" - the chances/time/and amount of discovery will rise up greatly - we will work and live for millions/billions of years until we have found all there is to find, but I will make sure to always be thousands of years ahead of them "to keep the balance"
Once I have discovered everything to the core of their science - I will create new science/new worlds and so on and so on for all infinity
and I will connect the Countless universes to 1 source of ultimate energy "My own body/Mind"
No one, no world, no verses no unimaginable entity will be able to lift a finger without my knowledge and my full control if i so wish it - but I would rather give them free will to be honest.
Anyway this right here is only the beginning - this is when things will start get interesting "its the infinite worlds, each more beautiful, more unique and more Unimaginable Uncomprehendable than the other, that i would create thats going to be worth the eternal life"
i Will reach to what I call "the halls of truth" and Edit it Unlimited!!
Infinite numbers of Infinite kinds in Infinite times "and beyond" is what I want "The basic three dimensional Infinity I call it"