I've been delaying posting in this thread because of time constraints and wanting to make a decent post about it.
My thoughts on an ammo system for Xerath:
I'm actually quite afraid of moving Xerath's ult to an ammo system. In order to balance out the fact that Xerath can pop you with an ult charge almost whenever he wants, you would need to reduce something it does now, ie range/damage/number of max charges.
One thing I do like that the ammo system solves is Xerath feeling like a limp noodle whenever his ult is on cooldown. It would allow you to get an EQR combo off and harass your opponents more than once every 60 seconds (which to be fair is super short).
In regards to giving Xerath more poke via ammo system:
I actually don't think he's deficient at poking right now. Once you get your Q/W leveled up you can rattle off long range poke constantly. I think the usability of his W and passive need to be addressed before his ultimate.
Disclaimer: These are my personal thoughts.
Right now I feel that Xerath's biggest flaw is his lack of lane clearing, it takes a lot of gold and time before Xerath can clear an entire wave of creeps compared to other ap's. However with an ammo system he can choose to use a charge or two to clear the lane properly.
He is a champion that has more positioning options than any other champ in the game, however his skillshots are sometimes difficult to land or ineffective due to being so narrow. The delays, the unforgiving nature of such narrow skills and the inability to clear lanes effectively(wraiths take forever) are already huge disadvantages to him.
Can he not just have this one little gift to make him more of a champion and less of a gimped peasant?