If I were a dead leaf thou mightest bear (43)
Not only does this remind us that the speaker is obsessed with dead leaves, it reminds us that "leaf" could mean a page in a book as well as something that dropped from a tree. If the speaker is a poet, as he implies, then his obsession with "dead leaves" might have more to do with his own writing than with Nature.
Drive my dead thoughts over the universe Like withered leaves to quicken a new birth! (63-64)
Death always results in resurrection in this poem; even though the speaker doesn’t anticipate that he himself will be resurrected, he believes that spreading his own "dead thoughts" around will create an opportunity for new ideas to be developed.
If Winter comes, can Spring be far behind? (70)
It’s important to read this as an open question. For the West Wind, which is autumn’s breath and ushers in the winter, spring is always on its way; for the speaker, though, whose "Winter" is more metaphorical, it’s entirely possible that his ideas, or even the intellectual current of the world, could wither away and not be renewed. He hopes that the death of some ideas and philosophies means that we’re about to come up with some better ones – but he’s not certain.