Crossovers that MUST be

[Jorlem]

Thinking about it, you're right. Makoto was able to learn to do that dimension jumping thing eventually, so the spells/tech must be there somewhere. The bit with the Eye of God would have to stay fairly close to canon to preserve the closed loop, so I don't think Precia would end up using it, though she would definitely try.

Also, do think it would be reasonable for the Tome of the Night Sky to have been a prototype/proof of concept for Ifurita's technique copying ability? The Tome would have been modified once, into a researcher's tool, then again in an attempt to create something like Ifurita, resulting in it becoming the Book of Darkness. This also leaves open the possibility that the Tome's original programming still exists somewhere on El-Hazard.
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Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber."  --Krityan Elder, Tales of Vesperia.