I like to think of The Hobbit trilogy (flaws and all) as Bilbo's interpretation in his memoir, "There and Back Again." Conversely, The Fellowship of the Ring's opening reflects how others, like Galadriel, describe those events, explaining the differences in Bilbo's appearance. So, adding Martin Freeman into LOTR might not be necessary. He could be seen as Bilbo's romanticized alter ego, adding depth to both trilogies and portraying Bilbo as not only the hero of his adventure but also the storyteller crafting his own legend in Middle-earth.