The foreshadowing

This is one of the coolest connections to previous X-Men movies, simply because the foreshadowing was apparently resolved. In 2013’s The Wolverine, Yukio (Rila Fukushima) revealed how Logan would die:

I see you on your back, there’s blood everywhere. You’re holding your own heart in your hand. It’s not beating. (IMDB)

A later scene showed Wolverine performing surgery on himself to remove a parasite from his own heart.

However, this prophecy also had a parallel in Logan. And we might add that it was a beautiful and heart-breaking parallel. When Logan died, he was lying on his back, covered in blood, and holding the hand of his daughter, Laura. It can’t get more heartbreaking than that.

And, of course, James Mangold later revealed that this was the foreshadowing.