There are lots of ways to challenge yourself in flying.
Bad weather can make for interesting flying. When the weather is marginal, it can be a challenge to get where we want to go safely. Find some bad live weather and challenge yourself to fly VFR in marginal conditions. Plan your flight carefully and think about worst-case scenarios, alternates and fuel requirements.
Consider crossing a continent, crossing an ocean, or flying around the world. All of those things can be made more challenging by choosing less capable aircraft and flying VFR (particularly with live weather).
Take a bush trip in a bush plane. I don’t know about the Asobo Pilatius Porters, but the Milviz Porter is excellent. Try and squeeze into some jungle strips in New Guinea or try landing on the glaciers of Mt. Cook in New Zealand.
Buy some new scenery for airports, cities and regions. Pick up some new freeware on Flightsim.to. You can get all sorts of different types of scenery to breathe even more life into the world.
“Earn” a private pilot’s license (and more) following a fairly realistic curriculum. Check out VatStar. You can follow a fairly realistic path to earning various licenses and ratings. Keep going and earn an instrument rating.
Buy the Boeing Model 247 and once you can keep it flying without catching your engines on fire, learn how to navigate by flying the beam.
Try learning celestial navigation and then try to navigate to a small island in the Pacific using only fixes you take from a sextant.
Get the Stearman biplane and challenge yourself to a long cross-country tour. Since there are no navigation instruments, you’ll have to rely on pilotage and dead-reckoning.
Learn some aerobatics and/or do some “wild” flying like extreme low level, flying under bridges, between buildings etc.
Hope that helps/have fun!