Will GMs have the ability to create their own adventures and materia using the core rules or future source books?
There is plenty of hooks and advice in the core rules for creating your own adventures, including a random plot generator if you need something to spark ideas.
There is also the sizable canon of books out there to be mined for ideas or minor characters.
For future source books they have said there will be a mix of adventures of setting books. The only confirmed one so far is Heirs of Dune which is a boxed starter adventure to get groups up and running in no time.