Using the Rey/Luke plot from the TLJ as the framework for a series of flashbacks exploring the whole saga sounds amazing. Especially because it would supplement the saga neatly without replacing anything really. If I was to take a shot at it, I would try to strike a balance between flashbacks and our main story of Luke gathering his courage to escape his self imposed exile and restore hope once again.
So thinking about it from this angle, it really should start with the destruction of the Hosnian system. This sets up the story, the capital is destroyed, the FO rules with fear and all hope seems lost. All hope but for the lightsaber, so Rey follows BB-8 and takes off with Chewie to find Luke. Then we follow their story on Ach-To, intercut with frequent flashbacks to the PT, the OT and maybe some scenes explaining Kylo‘s relationship with Rey, Han and Luke. In the end Rey leaves Luke behind to face Kylo, Luke has his moment with Yoda and we can transition to Luke facing Vader and the Emperor. After the flashback Luke joins the resistance on Crait and distracts Kylo, so that the rebellion can live.
We close on Luke watching the sun set over Ach-To and the kids on Canto Bight spreading the legend of Luke Skywalker. Broom Boy leaves the building and gazes up to the stars, where in this moment the falcon and the civil fleet appear, restoring said hope to the galaxy.