Primordial Hydra seems like your kinda fatty. You seem to love hero's bane, and this guy has the same effect, but for free, with the added benefit of often starting out much, much larger. Trample also helps and is easy to hit since if you cast him for at least 5x, then he'll be with a xeno trigger.

The downside to the hydra is that it does depend on the counters. So yes he can double the power, but it won't stay and it'll Slowly get crazy: I like Heroes' Bane because I can make him a base power 12/12 by the second time he attacks. But, I do like primordial though. I may take out the Artisan of Kozilek for him

I have a xenagos deck too, and I love it. It might be my strongest deck. If you're running Hellkite Charger, why not add Nature's Will and go infinite.

Seize the Day is brutal, and for 7 mana you get 2 extra combats. Well worth it in my book.

I actually want to take Hellkite Charger out because when I get to 7 mana and cast him, most of the time I don't have enough mana for the extra combat step.

Also, you would have to activate Hellkite Charger before Nature's Will in order to go infinite anyways.

So the sequence would go "Pay 7, cast it. Attack for 10. Trigger, pay 7. Extra combat step. Deals damage. Triggers Nature's Will. Untap. So I would need 14 mana to go infinite. Doesn't seem worth it to me.