Instead of an unnecessary LotR flashback, the last Hobbit film should have ended with Bilbo planting that acorn. He would have planted it, walked into his Hobbit hole, and closed the door of Bag End.
The camera pans out and time has passed because the acorn has grown into a tree. The camera angles toward the tree and a young Frodo runs up to an older Bilbo sitting in the shade of the leaves.
Frodo sits down next to his Uncle and asks “Why do you sit under this tree so often?” And Bilbo gives his nephew a wistful smile. “Well, my dear Frodo, let me tell you a story…”
“…about an unexpected journey that took me a long way from home.” The camera moves out slowly and Bilbo’s voice fades as the camera pans out above the tree overlooking the Shire.