So first thing that comes to mind is... a timeturner accident in the Great Battle in the Hall of Mysteries end of year 5 for Hermione sends her back to Regulus' time... somehow in close proximity to him to force him to deal with her? What point in time is a good question - anywhere on that spectrum of 16-18... tho the age and place will present unique difficulties (like, lol, at Hogwarts, how to hide a girl in your dorm who's just shown up?! or as a Death Eater, what do you do with a Muggleborn on your door step?)

Maybe the best point in time is just after Kreacher's stint in the cave and before Regulus decides to got get himself killed?

Thoughts?