A canon AU would be an alternate universe or alternate timeline that is presented in the source material, for long enough that the characters have distinct differences to be considered separate characters from their original universe counterparts. An example would be the Avengers Ultimates, which is set in a different universe from the main Avengers continuity.
If you can give me a more specific idea of what you're looking for, I can tell you whether it would be acceptable or not.
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A canon AU would be an alternate universe or alternate timeline that is presented in the source material, for long enough that the characters have distinct differences to be considered separate characters from their original universe counterparts. An example would be the Avengers Ultimates, which is set in a different universe from the main Avengers continuity.
If you can give me a more specific idea of what you're looking for, I can tell you whether it would be acceptable or not.