What are these worlds that gamers play in other than stories that set the background for further creation? Those world-builders enable their creations to spur other people into creating. Tolkien and Lewis, while probably not sitting behind the keyboard themselves, would have been enchanted with the ideas of these games.
Creating is part of the image God that was imbued into humanity.
Lewis frequently gets lumped in with Tolkein, because the two were friends, colleagues, and theological sparring partners. However, Lewis wrote “The Lion The Witch and the Wardrobe” specifically as a Christian morality play for his niece. He later expanded the novel in to the Chronicles series, but his intent was, again, to make issues of Christian morality and theology accessible to young people. If you think that he would have been amused with adults obsessing over his fantasy or sci-fi universes, get the a copy of “Miracles,” “The Screwtape Letters,” or (especially) “Mere Christianity,” and start reading carefully.
The bulk of publications from both men was heavily non-fiction, and it is clear that that is what they would have chosen to be remembered for.