First thing, this story has been finished on FF.net since May 22. It's 44 chapters and 345,675 words long, if someone wants to edit the OP. Then, on to the review:

This story is exactly what a "what if" fic should be: the changes can all be reasonably traced back to that one divergence point and the characters don't suddenly do everything differently with no reason. I'm annoyed at all the AUs advertising themselves as simple "what if", this one was a real breath of air.

It's basically the sister route of Heaven's Feel, the way UBW and Fate are sisters routes of their own. Chaos Theory could be the 4th route of the original game, with just an additional choice to follow Ilya at the beginning, it just lacks a sound track (which is completely unrelated to the fact I've had the Emiya song on loop for 3 days. Totally unrelated). It's like the pure awesomeness that Heaven's Feel was at moments, but without the worst parts and with even more Ilya. And Saber getting what she deserved. And just the right mentions of the other series of the Fate franchise.

Granted, that final arc turned a little bit too Grand Order-y to me and was less perfect than the rest of the story, but the first 30 or so chapters were at least a 10, so I still feel as if I'm too harsh by giving it a 5/5 in general. It's pure enjoyable fun, the characterizations are perfect, the story believable in the limits of canon, and the final as epic as you could ever want it. Couldn't recommend enough to anyone with the most general interest in Fate series.