Oh dear, this is magnificent! I love so many things about this. Magic having it's limitations and so much more that I can't even expound upon in one sitting, but mostly it was the time travel. I love how it didn't matter if Teddy was careful not step on the wrong bug, just his interactions can cause drastic change. And when he does try to play with fate, it doesn't work out precisely as he had planned.
Sequel? I hate to press, but I see it mentioned in previous comments and I am quite greedy when it comes to good fic.