How to tell a girl you'll accept her in the future?

[man]

I got rejected by a girl and decided to be friends. How do I go about saying

"if you ever change your mind in the future, let me know"

or any other variation that doesnt show weakness?

Also how close friends should I be? best friends or casual?

Also she rejected me because she's really content on this guy shes been "talking to" for 5 months.