So I guess this flashback will likely show the events that occurred between when Rumple sent Cinderella to the ball and Cinderella’s wedding. We never did get to see the actual ball, Cinderella and Prince Thomas falling in love, the iconic search with the glass slipper, nor even Cinderella directly interacting with her stepfamily onscreen.
If it’s not this…maybe perhaps there were a few fairies who blame Cinderella for one of their own getting killed by Rumplestilskin, and they are the ones who took Thomas to the Land of Untold Stories, in hopes that the tale for him and Cinderella never reaches a happy ending. It’d parallel a fairy originally trying to help Cinderella find her happy ending to fairies trying to ruin her happy ending.
"If you go as far as you can see...you will then see enough to go even further." - Finn Balor