Don’t worry too much about finding a worthy opponent. If you’re good enough and famous enough, a nemesis will find you. While waiting for your opposition to arrive, defend citizens from lower grade criminals. It’s a great way to practice your crime fighting style and gain the respect and adoration of the public. Once you gain a rapport, two things are bound to happen: 1) the public will buy more of your merchandise and 2) meaner people will show up anxious for a tussle.
If your business is lagging, take a look at the crime rates in your area. Are they high enough to sustain a healthy superhero income? If not, try moving to where the action is. Let local law enforcement deal with the small fries, and seek bigger bait yourself. Gotham is taken, but there are hundreds of unclaimed American cities just like it.