How can I make AD more robust?

My definition of robust in reference to AD would be the frequency and ease of replication across sites and removal of orphaned objects (excluding accounts) and I do agree with you on the ease of recovery
from disaster scenarios.

I'm not sure there's a lot to be done besides creating additional domain controllers. Maybe if you could explain a bit more about what you're trying to achieve...

Originally the question was asked during an interview and I just wanted to know if there was more that could be done in addition to what that has been mentioned previously.

Hmmm. Did you not get the job? Is this the question that gave you the most trouble?

As a hypothetical, asked in an interview, I'd guess that they were probing for more general knowledge about AD and the underlying mechanisms it uses for replication and protection in an environment subject to many possible types of failure. But why bring up orphaned objects and accounts? It occasionally happens that an interviewer asks a poorly thought out question; you have to be a mind reader to get what they're really after.

Thank you very much for your in-put