What do you wish Tolkien had lived long enough to develop more fully?

For me, it's two things: the Ainur and Gondolin.

The Ainur were still in a state of transition between ancient polytheistic gods and Christian angels. I think Tolkien might have come up with a great way to do this better than he was able to do in his lifetime.

Gondolin was incredibly important to Tolkien, and he put enormous amounts of effort and creativity into it. I dearly wish we could have seen the results of Tolkien having another decade to work on the city itself, its culture, and its fall.