Is this a good idea?
If you write fan fiction, this is a great idea. Personally, I am not a fan of anything Amazon can obtain a license for, and, even if they did, I'd have a really hard time writing fan fiction for The Complete Works of Shakespeare. The only drawback I can see to Amazon's latest bid to take over the world (mwahahahaha) is that getting a license for, let's say, Star Trek, will be nigh on impossible. In other words, die-hard fans of really popular shows still won't have an outlet for their frustrated desire to improve upon, continue, elaborate on and otherwise embellish the plots/characters of anything that could be truly lucrative. (Trust me, if they did, Spike would still be alive.)
So, if you inhabit any of the "Worlds" Amazon is currently allowing you to live on, go for it.