There's not much new that one can currently do to add to the variety of possible time travel metaphysics explored in literature. Try-to-improve-stuff-make-other-stuff-much-worse is a pretty reliable trope by this point. Stephen King's 11/22/63 (in addition to being a great novel) is still philosophically interesting though.
What 11/22/63 makes distressingly clear is that in King's universe Leibniz is correct that this is the best of all possible worlds. When the time-traveller goes back in time the universe supernaturally conspires (through standard horror movie tropes such as inanimate objects behaving as if they are agentive, people acting possessed, etc.) to prevent him from altering anything that would change the course of history or to put history back on track once he's altered it. When the time traveller finally beats the universe at this game, the results are catastrophic, and he then becomes part of the universe horrifically setting itself aright.
For the poor time-traveller it is impossible* to improve history. The thought that one might improve history is only the result of ignorance.
Again, this is not new stuff. It's one response to the problem of evil. What's new in King's book is that he shows Leibniz's conclusion to be itself horrific. It would be madness to worship whatever supernatural forces ensure that this is the best possible world. By realistically portraying the moral psychology of humans buffeted by these forces (made visible to humans by time travel) King is able to demolish the Leibnizian intuition more effectively than Voltaire.
Reformation Theology can be summed up by three Gs: Guilt, Grace, Gratitude. For King, if this really were the best possible world, that's all the more reason to be ungrateful. We're not faced with a good God who can't do better than this (is that process theology?). But rather horrific processes that we can't understand making it impossible that things could be better.
*To be fair, horror impossibility is not the kind of impossibility philosophers normally think about. In horror the impossible is sometimes actual. Noel Carroll comes closest to explaining how this works. Neal Hebert and I tried to expand on Carroll's analysis, but I think our account was overly epistemic. I'm going to teach Harman's book on Lovecraft next year and hopefully be able to rethink this.]