League of Legends (LoL) Question: Is there a way to request guides?

We don't have an official program for this kind of thing. Your best bets are probably:

• Make a thread in the Build & Guide Discussion subforum and see if there's any interest there.
• Contact a guide author you trust and see if they'd be willing to write a guide for a given champion.
• Ask a player you know (someone who mains the champion) to write a guide for the champion. Maybe even co-author it with them.

The third method has worked for me once or twice. If you want to PM an established guide author, you'll probably have to find one that's still putting out new guides.