Hehe, looks like Domi has been bitten by the Community bugā¦ Congratulations! 
Back on the old Forum Justin used to host monthly āsong competitionsā, occasionally themed. They were more good fun than anything else, but a lot of work for the team.
You learn a lot from collaborating with others and have taken the first step with your post. Even if there is little demand for a āchallengeā Iād be surprised if there wasnāt anyone up for helping out with lyrics or music on any project you fancy tackling.
Iām afraid Iām embroiled in another āfamily & friendsā project Iām putting together, but Iām sure someone else will bite. Also, donāt be afraid to message folk directly. The worst they can do is politely decline 
Your Beatles anecdote reminds me of this one I did along similar lines
If you are bored, Richard curated a very long post of an āaccidentalā collaboration on the old forum and posted it here 
Good luck! 