It's happened before, but it doesn't happen often. And it'll go down soon.
Will Ospreay announced that he will be facing Pete Dunne at IPW's Magnificent Seven event on January 17. He tweeted the following below:
It has happened before, in fact. Then WWE champion Hulk Hogan faced off against then IWGP champion The Great Muta back in 1993. But with Dunne being able to work independent shows as a champion under the WWE umbrella, the situation's never been available for a WWE champion to face a champion from New Japan until now.
Ospreay regained the IWGP Jr. Heavyweight championship at Wrestle Kingdom 12 last week. Pete Dunne has been WWE UK champion since May 20 of last year.