I don't see the point in making an AU where all the characters are normal living in the modern world. If it's a prompt that requests a modern AU, that's different, the prompter is requesting it. I know some people really like AUs, and I'd guess that keeping the characters recognizable and in character is an interesting writing challenge. But I really don't understand why someone would take a prompt that doesn't ask for a modern AU at all and fill it with a modern AU.