Once upon a time there was Ushio and Tora, a shonen from the '90s that you may or may not know. In it, our two main characters are at some point sent into the past, to a point in time where an event important for the plot is supposed to happen, to live the flashback themselves. They meet some people, befriend them, and eventually become part of the reason why the aforementioned event takes place, mainly because they are a source of motivation for those friends. They even have a brief encounter with Hakumen, the main villain of the series, that explains why she remembers them when she sees them in present day.
Back in the day I thought this was a very original way to tell a flashback and I have never seen such a technique after that...until Undead Unluck. I suppose once every 30 years is fair enough, and in both occasions it was impactful and a great flashback arc, different from the standard.