Hiking off trail in general will give you an experience that is more difficult. You don't have to peakbag to get a harder experience.
At least in Colorado, I usually avoid the high points (14ers) as they are too crowded and even "off trail" experiences have social trails. Of course, the trail less 13ers are another ball of wax. They are usually more difficult than the majority of the 14ers. And definitely less crowded.
Hiking for difficulty is not why I hike. I hike because I enjoy it. By coincidence, some of the hardest hikes are also sometimes the most enjoyable. And sometimes these hikes do not end at a peak, but a remote valley or a trail-less ridge
YMMV