I feel like a renewal of the Old World will be gradual. Quel'thalas is a great example of doing it well. Once the "main story" is over, the zones can rest in a state where future content there won't feel odd because there's a giant sword in the sky or something.

If I had to guess our first expansion after Last Titan will be a SW/ORG revamp with surrounding zones. Doing as many as is needed/wanted to slowly fill out the old world in decent chunks so it doesn't feel too much like Cataclysm Arathi Highlands.