Willie used to be used more as a nickname but seems out of date rather like
Dick or
Abe. I think it is still occasionally used in the south, but it is rarely used on its own.
Willy (that spelling) is occasionally used to refer to a man's privates.
Willi makes me think of getting the willies (jitters).
Fritz - too stereotypically
German.
Bent,
Bente -
Bent is to wordy and broken.
Brent works okay though.
Fanny - No. It is slang for a bottom.
Lion (LEE-awn) - If you want that pronunciation, then
Leon would be a better bet.
Arian (A-ree-an) - sounds too much like
Aryan, which you wouldn't want to use especially if you are
German. (I'm sure you know this.)
Christian (KRIS-tee-an) - It would probably end up being pronounced in the English way. It isn't my cup of tea but isn't unusual.