I was going to avoid the "Where in Time...?" bit due to it being a lost Magic in Nasuverse, as part of making the crossover fit together. Though time travel hax would be one way to bring about a Utopia ending, as long as its set up to be a major effort useable once or twice only - make a huge mess of the 5th war, then protag party jumps back to the 4th and shuts it down in time to save Irisviel, get kid Sakura out of the rapeworm pit, etc. But, Shiro's position against time travel is clear.
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"Anko, what you do in your free time is your own choice. Use it wisely. And if you do not use it wisely, make sure you thoroughly enjoy whatever unwise thing you are doing." - HymnOfRagnorok as Orochimaru at SpaceBattles
woot Med. Eng., verb, 1st & 3rd pers. prsnt. sg. know, knows
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"Anko, what you do in your free time is your own choice. Use it wisely. And if you do not use it wisely, make sure you thoroughly enjoy whatever unwise thing you are doing." - HymnOfRagnorok as Orochimaru at SpaceBattles
woot Med. Eng., verb, 1st & 3rd pers. prsnt. sg. know, knows