Add a Friends Option to Wall Post Options

Hello! I had a small idea that could possibly benefit the community through wall posts. For wall posts, I thought maybe a "Friends" option could work by only subscribers who subscribe to the poster can see the post. In other words, reciprocal subscribers! That's all there is to it! Below is a simple mock-up image I created to visualize the idea.

Update: Thank you, AvatarKage, for bringing this to my attention! Essentially, this post is more focused on adding a "mutuals" option rather than requesting a 'friends' option for wall posts. However, I still believe adding a "mutuals" and "friends" option (as well as a system to request such friends) would be a great addition to the site as a whole.