The most common way to add difficulty to a climbing route is to climb it without using all the available features in the rock.
There's a popular top roping crag in Waterton Lakes National Park that sees a lot of traffic. To mix things up a bit, the locals will climb the routes without using specific types of features, such as no horizontal cracks, no closed hold jugs, no pinches, etc. Or they'll play a variation of the climbing game "take-away" where the climbers will choose holds to eliminate from the route.
Another fun variation is to climb as dynamically as possible, skipping past entire sections of the route, or seeing who can add in the biggest dyno possible.
Once you've dyno'd all you can, you can try to campus as much of the route as possiple, or add in ridiculous beta, like unnecessary heel hooks or bat hangs.
After you've climbed a route enough times, you can start climbing it as fast as you can, to see who can red-point the route the fastest. Alternatively you could try to see who can climb the route with the fewest number of moves.
The most daring of challenges is to go full Honnold on the route and free-solo it (For expert climbers only! If you can't climb a route with ease, knowing for a surety you can climb it flawlessly without risk of taking a fall, then you should never attempt it).