Add uid to existing menu link
As i can see from the menu token that the module has a pre-released version for drupal 8 as development release, so maybe you could try it out or at least search in its code to find a way to accomplish your needs.

I have recently built a drupal 7 website and i have used the menu token to create a hover over drop down usermenu with username and profile picture.

See image below (i drawed in black on top of it because the site in not yet online)


If you like, i can right back small tutorial on how i achieved that !