I like Seth and Owen They both go nicely with William too..or even Willem or Wilhelm.

Milo
Henry
Tristan
Hamish
Reuben

European is very broad so Im going with Dutch/German based on your post.
I like Jurgen and Gerhardt but ithey're a bit different for most people
Hans
Lars
Kees
Gregor
Hilton

Hmm.. not sure..