I have read that J.R.R.Tolkien's Numenorian legend in the form of The Lost Road and the Out of the Silent Planet series by C.S.Lewis took shape from deciding on topics in their writing group, the Inklings.
Tolkien's topic was time travel, and Lewis' topic was space travel.
Were any of the other members involved with topics, and if so, were there any other literary contributions that came of them?