While noting the observation that Tolkien always planned on having Frodo survive, I've always thought an interesting plot development (and one taking away a criticism by some at the time the book came out that just about all the major characters survive) would have to had Frodo die from Shelob's sting and Sam then carry the Ring the rest of the way---just him and Gollum.
Biggest minus might have been having to rework the bittersweet ending.