A lot of things we live with have significant technical debt because they were designed and implemented without modern knowledge and understanding.
Knowing what we know now, in the present moment, what would you be interested in fundamentally redesigning if you could?
This does NOT have to be technology related, by the way, though it certainly can (anyone want to talk about redesigning usernames and passwords -- please?). It can pretty much be anything: NASCAR races, art criticism, specific social norms, sunglasses, etc.
In your explanation, don't just share what you're interested in tearing down, but how you would rebuild it for the better. What improvements would your methods bring to the table?